diff --git a/Source/AudibleUtilities/Mkb79Auth.cs b/Source/AudibleUtilities/Mkb79Auth.cs
index 4731a806..e58e6041 100644
--- a/Source/AudibleUtilities/Mkb79Auth.cs
+++ b/Source/AudibleUtilities/Mkb79Auth.cs
@@ -237,6 +237,12 @@ public partial class Mkb79Auth
 		return account;
 	}
 
+	/// <summary>
+	/// The exported file names one marketplace - the one this account is registered with - because that is all
+	/// the format holds: a single <c>locale_code</c> alongside a single device registration. audible-cli switches
+	/// marketplaces on its own from those same tokens, so nothing is lost to it. Any additional marketplaces
+	/// Libation reads for this account are its own bookkeeping and have no slot here.
+	/// </summary>
 	public static Mkb79Auth FromAccount(Account account)
 		=> new()
 		{
diff --git a/Source/AudibleUtilities/Mkb79AuthImporter.cs b/Source/AudibleUtilities/Mkb79AuthImporter.cs
index 82b21ccb..a4227dba 100644
--- a/Source/AudibleUtilities/Mkb79AuthImporter.cs
+++ b/Source/AudibleUtilities/Mkb79AuthImporter.cs
@@ -1,4 +1,4 @@
-using System.Linq;
+using System;
 using System.Threading.Tasks;
 
 namespace AudibleUtilities;
@@ -10,10 +10,37 @@ public enum Mkb79ImportOutcome
 	InvalidFile,
 }
 
-public sealed record Mkb79ImportResult(Mkb79ImportOutcome Outcome, Account? Account = null, string? Message = null);
+/// <param name="ClaimedBy">
+/// For <see cref="Mkb79ImportOutcome.DuplicateAccount"/>, the account already scanning that marketplace. It may
+/// be one registered with it, or one carrying it as an additional marketplace.
+/// </param>
+public sealed record Mkb79ImportResult(
+	Mkb79ImportOutcome Outcome,
+	Account? Account = null,
+	string? Message = null,
+	Account? ClaimedBy = null);
 
 public static class Mkb79AuthImporter
 {
+	/// <summary>
+	/// Why a duplicate import was refused, in the same words everywhere it is refused. Naming the account that
+	/// already reads the marketplace matters now that it need not be a row registered with it - it may be one
+	/// reading it as an additional marketplace, which is not obvious from the accounts grid.
+	/// </summary>
+	public static string DuplicateMessage(Mkb79ImportResult result)
+	{
+		var locale = result.Account?.Locale?.Name ?? "[unknown]";
+
+		if (result.ClaimedBy is { } claimedBy && claimedBy.Locale?.Name != locale)
+			return $"The '{locale}' marketplace is already scanned by the account "
+				+ $"{AccountCredentialStatus.FormatAccountLabel(claimedBy)}, as an additional marketplace. "
+				+ "Nothing was imported.";
+
+		return "An account with that account id and country already exists."
+			+ $"{Environment.NewLine}Account ID: {result.Account?.AccountId}"
+			+ $"{Environment.NewLine}Country: {locale}";
+	}
+
 	/// <summary>
 	/// Deserialize mkb79/audible-cli JSON, refresh tokens, and add the account if not already present.
 	/// </summary>
@@ -32,11 +59,12 @@ public static class Mkb79AuthImporter
 
 		using var persister = AudibleApiStorage.GetAccountsSettingsPersister();
 
-		if (persister.AccountsSettings.Accounts.Any(a =>
-				a.AccountId == account.AccountId && a.IdentityTokens?.Locale.Name == account.Locale?.Name))
-		{
-			return new Mkb79ImportResult(Mkb79ImportOutcome.DuplicateAccount, account);
-		}
+		// An mkb79 file names one marketplace, and a marketplace can only be scanned by one account. Ask about
+		// every claim on it, not just registrations: an existing account may already be reading this marketplace
+		// as an additional one, in which case importing would scan it twice.
+		var claimedBy = persister.AccountsSettings.GetAccountClaimingMarketplace(account.AccountId, account.Locale?.Name);
+		if (claimedBy is not null)
+			return new Mkb79ImportResult(Mkb79ImportOutcome.DuplicateAccount, account, ClaimedBy: claimedBy);
 
 		persister.AccountsSettings.Add(account);
 		return new Mkb79ImportResult(Mkb79ImportOutcome.Success, account);
diff --git a/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml b/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml
index c0cf27b4..1dc81128 100644
--- a/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml
+++ b/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml
@@ -87,6 +87,23 @@
 					</DataGridTemplateColumn.CellTemplate>
 				</DataGridTemplateColumn>
 
+				<DataGridTemplateColumn Width="Auto" Header="Marketplaces">
+					<DataGridTemplateColumn.CellTemplate>
+						<DataTemplate>
+
+							<Button
+								Content="{Binding MarketplacesButtonText}"
+								VerticalAlignment="Stretch"
+								HorizontalAlignment="Stretch"
+								HorizontalContentAlignment="Center"
+								IsEnabled="{Binding CanCheckMarketplaces}"
+								ToolTip.Tip="{Binding MarketplacesButtonToolTip}"
+								Click="MarketplacesButton_Clicked" />
+
+						</DataTemplate>
+					</DataGridTemplateColumn.CellTemplate>
+				</DataGridTemplateColumn>
+
 				<DataGridTextColumn
 					Width="Auto"
 					Binding="{Binding AccountName, Mode=TwoWay}"
diff --git a/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml.cs b/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml.cs
index b55dc3d8..6a86d50f 100644
--- a/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml.cs
+++ b/Source/LibationAvalonia/Dialogs/AccountsDialog.axaml.cs
@@ -3,6 +3,7 @@ using AudibleUtilities;
 using Avalonia.Collections;
 using Avalonia.Controls;
 using Avalonia.Platform.Storage;
+using LibationUiBase;
 using LibationUiBase.Forms;
 using ReactiveUI;
 using System;
@@ -45,6 +46,12 @@ public partial class AccountsDialog : DialogWindow
 		public string? AccountName { get; set; }
 		public bool IsDefault => string.IsNullOrEmpty(AccountId);
 
+		/// <summary>
+		/// Marketplaces beyond <see cref="SelectedLocale"/> that this account should also scan. Edited by the
+		/// marketplaces dialog and written on save, like every other field here.
+		/// </summary>
+		public List<string> AdditionalLocaleNames { get; } = new();
+
 		public bool CanExport
 		{
 			get => field;
@@ -56,6 +63,17 @@ public partial class AccountsDialog : DialogWindow
 				? "Export account authorization to audible-cli"
 				: "Authenticate this account (e.g. library scan) before exporting to audible-cli.";
 
+		/// <summary>
+		/// Checking other marketplaces uses this account's stored credentials, so it needs the same thing an
+		/// export does: an account that has logged in at least once.
+		/// </summary>
+		public bool CanCheckMarketplaces => CanExport;
+
+		public string MarketplacesButtonText => MarketplacesUi.ButtonText(AdditionalLocaleNames.Count + 1);
+
+		public string MarketplacesButtonToolTip
+			=> CanCheckMarketplaces ? MarketplacesUi.ButtonToolTip : MarketplacesUi.NotAuthenticatedToolTip;
+
 		public AccountDto() => RefreshCanExport();
 
 		public AccountDto(Account account)
@@ -64,13 +82,28 @@ public partial class AccountsDialog : DialogWindow
 			AccountId = account.AccountId;
 			SelectedLocale = Locales.Single(l => l.Name == account.Locale?.Name);
 			AccountName = account.AccountName;
+			AdditionalLocaleNames.AddRange(account.AdditionalLocales.Select(l => l.Name));
 			RefreshCanExportFromAccount(account);
 		}
 
+		public void SetAdditionalLocaleNames(IEnumerable<string> localeNames)
+		{
+			AdditionalLocaleNames.Clear();
+			AdditionalLocaleNames.AddRange(localeNames);
+			this.RaisePropertyChanged(nameof(MarketplacesButtonText));
+		}
+
 		private void RefreshCanExportFromAccount(Account account)
 		{
 			CanExport = account.IdentityTokens?.IsValid == true;
+			RaiseDerivedFromCanExport();
+		}
+
+		private void RaiseDerivedFromCanExport()
+		{
 			this.RaisePropertyChanged(nameof(ExportButtonToolTip));
+			this.RaisePropertyChanged(nameof(CanCheckMarketplaces));
+			this.RaisePropertyChanged(nameof(MarketplacesButtonToolTip));
 		}
 
 		private void RefreshCanExport()
@@ -78,7 +111,7 @@ public partial class AccountsDialog : DialogWindow
 			if (string.IsNullOrEmpty(AccountId) || SelectedLocale is null)
 			{
 				CanExport = false;
-				this.RaisePropertyChanged(nameof(ExportButtonToolTip));
+				RaiseDerivedFromCanExport();
 				return;
 			}
 
@@ -86,7 +119,7 @@ public partial class AccountsDialog : DialogWindow
 			var account = persister.AccountsSettings.Accounts.FirstOrDefault(a =>
 				a.AccountId == AccountId && a.Locale?.Name == SelectedLocale.Name);
 			CanExport = account?.IdentityTokens?.IsValid == true;
-			this.RaisePropertyChanged(nameof(ExportButtonToolTip));
+			RaiseDerivedFromCanExport();
 		}
 	}
 
@@ -174,9 +207,9 @@ public partial class AccountsDialog : DialogWindow
 				return;
 			}
 
-			if (importResult.Outcome is Mkb79ImportOutcome.DuplicateAccount && importResult.Account is { } dup)
+			if (importResult.Outcome is Mkb79ImportOutcome.DuplicateAccount && importResult.Account is not null)
 			{
-				await MessageBox.Show(this, $"An account with that account id and country already exists.\r\n\r\nAccount ID: {dup.AccountId}\r\nCountry: {dup.Locale?.Name}", "Cannot Add Duplicate Account");
+				await MessageBox.Show(this, Mkb79AuthImporter.DuplicateMessage(importResult), "Cannot Add Duplicate Account");
 				return;
 			}
 
@@ -199,6 +232,29 @@ public partial class AccountsDialog : DialogWindow
 			Export(acc);
 	}
 
+	public async void MarketplacesButton_Clicked(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+	{
+		if (e.Source is not Button btn || btn.DataContext is not AccountDto acc)
+			return;
+
+		// the probe speaks to Audible with this account's stored credentials, so it needs the saved account,
+		// not the grid's copy of it
+		using var persister = AudibleApiStorage.GetAccountsSettingsPersister();
+		var account = persister.AccountsSettings.Accounts.FirstOrDefault(a =>
+			a.AccountId == acc.AccountId && a.Locale?.Name == acc.SelectedLocale?.Name);
+
+		if (account is null || account.IdentityTokens?.IsValid != true)
+		{
+			await MessageBox.Show(this, MarketplacesUi.NotAuthenticatedToolTip, "Account Not Authenticated");
+			return;
+		}
+
+		var dialog = new MarketplacesDialog(account, persister.AccountsSettings, acc.AdditionalLocaleNames);
+
+		if (await dialog.ShowDialog<DialogResult>(this) == DialogResult.OK)
+			acc.SetAdditionalLocaleNames(dialog.SelectedAdditionalLocaleNames);
+	}
+
 	protected override async Task SaveAndCloseAsync()
 	{
 		try
@@ -246,6 +302,7 @@ public partial class AccountsDialog : DialogWindow
 		}
 
 		// upsert each. validation occurs through Account and AccountsSettings
+		var upserted = new List<(AccountDto Dto, Account Account)>();
 		foreach (var dto in Accounts.Where(a => a.AccountId is not null))
 		{
 			var acct = accountsSettings.Upsert(dto.AccountId!, dto.SelectedLocale?.Name);
@@ -254,7 +311,15 @@ public partial class AccountsDialog : DialogWindow
 				= string.IsNullOrWhiteSpace(dto.AccountName)
 				? $"{dto.AccountId} - {dto.SelectedLocale?.Name}"
 				: dto.AccountName.Trim();
+
+			// drop every marketplace before assigning any, so that moving one from one account to another in a
+			// single sitting cannot trip the "no two accounts scan one marketplace" rule halfway through
+			acct.SetAdditionalMarketplaces([]);
+			upserted.Add((dto, acct));
 		}
+
+		foreach (var (dto, acct) in upserted)
+			acct.SetAdditionalMarketplaces(dto.AdditionalLocaleNames);
 	}
 	private async Task<bool> inputIsValid()
 	{
diff --git a/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml b/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml
new file mode 100644
index 00000000..17023ff5
--- /dev/null
+++ b/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml
@@ -0,0 +1,81 @@
+<Window
+	xmlns="https://github.com/avaloniaui"
+	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	mc:Ignorable="d"
+	d:DesignWidth="560" d:DesignHeight="520"
+	MinWidth="400" MinHeight="360"
+	Width="560" Height="520"
+	x:Class="LibationAvalonia.Dialogs.MarketplacesDialog"
+	xmlns:dialogs="clr-namespace:LibationAvalonia.Dialogs"
+	x:DataType="dialogs:MarketplacesDialog"
+	x:CompileBindings="True"
+	Title="Marketplaces"
+	WindowStartupLocation="CenterOwner">
+
+	<Grid
+		ColumnDefinitions="Auto,*,Auto"
+		RowDefinitions="Auto,Auto,*,Auto,Auto"
+		Margin="10">
+
+		<TextBlock
+			Grid.Row="0"
+			Grid.Column="0"
+			Grid.ColumnSpan="3"
+			TextWrapping="Wrap"
+			Text="{Binding Intro}" />
+
+		<TextBlock
+			Grid.Row="1"
+			Grid.Column="0"
+			Grid.ColumnSpan="3"
+			Margin="0,10,0,0"
+			FontWeight="Bold"
+			Text="{Binding AccountLabel}" />
+
+		<ListBox
+			Grid.Row="2"
+			Grid.Column="0"
+			Grid.ColumnSpan="3"
+			Margin="0,10"
+			Name="lbMarketplaces"
+			ItemsSource="{Binding Marketplaces}">
+			<ListBox.ItemTemplate>
+				<DataTemplate>
+					<CheckBox
+						IsChecked="{Binding IsChecked, Mode=TwoWay}"
+						IsEnabled="{Binding CanCheck}"
+						ToolTip.Tip="{Binding ToolTip}">
+						<TextBlock Text="{Binding Text}" />
+					</CheckBox>
+				</DataTemplate>
+			</ListBox.ItemTemplate>
+		</ListBox>
+
+		<TextBlock
+			Grid.Row="3"
+			Grid.Column="0"
+			Grid.ColumnSpan="3"
+			TextWrapping="Wrap"
+			Margin="0,0,0,10"
+			Name="StatusTextBlock" />
+
+		<Button
+			Grid.Row="4"
+			Grid.Column="0"
+			Padding="20,6"
+			Name="CheckButton"
+			Content="{Binding CheckButtonText}"
+			Click="CheckButton_Clicked" />
+
+		<Button
+			Grid.Row="4"
+			Grid.Column="2"
+			Classes="SaveButton"
+			HorizontalAlignment="Right"
+			Content="Save"
+			Name="SaveButton"
+			Command="{Binding SaveAndClose}" />
+	</Grid>
+</Window>
diff --git a/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml.cs b/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml.cs
new file mode 100644
index 00000000..599a215d
--- /dev/null
+++ b/Source/LibationAvalonia/Dialogs/MarketplacesDialog.axaml.cs
@@ -0,0 +1,148 @@
+using AudibleApi;
+using AudibleUtilities;
+using Avalonia.Collections;
+using LibationUiBase;
+using ReactiveUI;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace LibationAvalonia.Dialogs;
+
+/// <summary>
+/// Which Audible marketplaces one account should read. Opened from the accounts grid, and only for an account
+/// that has already logged in - the check is made with that account's own credentials.
+/// </summary>
+public partial class MarketplacesDialog : DialogWindow
+{
+	public string Intro => MarketplacesUi.Intro;
+	public string CheckButtonText => MarketplacesUi.CheckButton;
+	public string AccountLabel { get; } = "";
+
+	public AvaloniaList<ListItem> Marketplaces { get; } = new();
+
+	/// <summary>The additional marketplaces the user checked. The registered one is never among them.</summary>
+	public IReadOnlyList<string> SelectedAdditionalLocaleNames
+		=> Marketplaces
+			.Where(m => m.IsChecked && m.CanCheck)
+			.Select(m => m.Locale.Name)
+			.ToList();
+
+	public class ListItem : ViewModels.ViewModelBase
+	{
+		public ListItem(Locale locale, string text, bool isChecked, bool canCheck, string? toolTip = null)
+		{
+			Locale = locale;
+			Text = text;
+			IsChecked = isChecked;
+			CanCheck = canCheck;
+			ToolTip = toolTip;
+		}
+
+		public Locale Locale { get; }
+		public string Text
+		{
+			get => field;
+			set => this.RaiseAndSetIfChanged(ref field, value);
+		}
+		public bool IsChecked
+		{
+			get => field;
+			set => this.RaiseAndSetIfChanged(ref field, value);
+		}
+		public bool CanCheck
+		{
+			get => field;
+			set => this.RaiseAndSetIfChanged(ref field, value);
+		}
+		public string? ToolTip { get; }
+		public override string ToString() => Text;
+	}
+
+	private readonly Account? account;
+	private readonly AccountsSettings? accountsSettings;
+
+	// parameterless ctor for the axaml designer
+	public MarketplacesDialog()
+	{
+		InitializeComponent();
+		DataContext = this;
+	}
+
+	/// <param name="selectedAdditionalLocaleNames">
+	/// What the accounts grid currently shows for this account, which may not yet be saved.
+	/// </param>
+	public MarketplacesDialog(Account account, AccountsSettings accountsSettings, IEnumerable<string> selectedAdditionalLocaleNames)
+	{
+		InitializeComponent();
+
+		this.account = account;
+		this.accountsSettings = accountsSettings;
+
+		AccountLabel = AccountCredentialStatus.FormatAccountLabel(account);
+
+		var selected = selectedAdditionalLocaleNames.ToHashSet();
+
+		// list every candidate up front, so the dialog is a full picture before anything is asked of Audible
+		foreach (var locale in MarketplaceProbe.CandidateLocales(account))
+		{
+			var isRegistered = locale.Name == account.Locale?.Name;
+
+			Marketplaces.Add(new ListItem(
+				locale,
+				isRegistered
+					? $"{locale.Name} - this account's own marketplace"
+					: locale.Name,
+				isChecked: isRegistered || selected.Contains(locale.Name),
+				canCheck: !isRegistered,
+				toolTip: isRegistered ? "Always scanned. This is where the account is registered." : null));
+		}
+
+		StatusTextBlock.Text = MarketplacesUi.ButtonToolTip;
+		ControlToFocusOnShow = CheckButton;
+		DataContext = this;
+	}
+
+	public async void CheckButton_Clicked(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
+		=> await ProbeAsync();
+
+	public async Task ProbeAsync()
+	{
+		if (account is null || accountsSettings is null)
+			return;
+
+		CheckButton.IsEnabled = false;
+		StatusTextBlock.Text = MarketplacesUi.Checking;
+
+		var results = new List<MarketplaceProbeResult>();
+
+		try
+		{
+			await foreach (var result in MarketplaceProbe.ProbeAsync(account, accountsSettings))
+			{
+				results.Add(result);
+
+				if (Marketplaces.FirstOrDefault(m => m.Locale.Name == result.Locale.Name) is not { } item)
+					continue;
+
+				item.Text = MarketplacesUi.ResultText(result);
+
+				// a marketplace another account already scans must not be checkable here: two rows scanning one
+				// marketplace would import it twice
+				if (result.Outcome is MarketplaceProbeOutcome.ScannedByAnotherAccount or MarketplaceProbeOutcome.Failed)
+					item.CanCheck = false;
+
+				if (result.Outcome is MarketplaceProbeOutcome.TitlesFound)
+					item.IsChecked = true;
+			}
+
+			StatusTextBlock.Text = MarketplacesUi.Summary(results);
+		}
+		finally
+		{
+			CheckButton.IsEnabled = true;
+		}
+	}
+
+	public new void SaveAndClose() => base.SaveAndClose();
+}
diff --git a/Source/LibationAvalonia/Dialogs/ScanAccountsDialog.axaml.cs b/Source/LibationAvalonia/Dialogs/ScanAccountsDialog.axaml.cs
index 34f34901..332d3fef 100644
--- a/Source/LibationAvalonia/Dialogs/ScanAccountsDialog.axaml.cs
+++ b/Source/LibationAvalonia/Dialogs/ScanAccountsDialog.axaml.cs
@@ -1,5 +1,6 @@
 using AudibleUtilities;
 using Avalonia.Collections;
+using LibationUiBase;
 using LibationUiBase.Forms;
 using System.Collections.Generic;
 using System.Linq;
@@ -17,7 +18,8 @@ public partial class ScanAccountsDialog : DialogWindow
 		{
 			Account = account;
 			IsChecked = account.LibraryScan;
-			Text = $"{account.AccountName} ({account.AccountId} - {account.Locale?.Name})";
+			// lists every marketplace: one checkbox here can scan more than one
+			Text = MarketplacesUi.ScanPickerText(account);
 		}
 		public Account Account { get; }
 		public string Text { get; }
diff --git a/Source/LibationCli/Options/ImportAccountOptions.cs b/Source/LibationCli/Options/ImportAccountOptions.cs
index 75f2ff9d..7d4b5d9d 100644
--- a/Source/LibationCli/Options/ImportAccountOptions.cs
+++ b/Source/LibationCli/Options/ImportAccountOptions.cs
@@ -59,10 +59,8 @@ internal class ImportAccountOptions : OptionsBase
 				Console.Error.WriteLine(result.Message ?? "Invalid import file.");
 				Environment.ExitCode = (int)ExitCode.RunTimeError;
 				return;
-			case Mkb79ImportOutcome.DuplicateAccount when result.Account is { } dup:
-				Console.Error.WriteLine(
-					$"An account with that account id and country already exists.{Environment.NewLine}"
-					+ $"Account ID: {dup.AccountId}{Environment.NewLine}Country: {dup.Locale?.Name}");
+			case Mkb79ImportOutcome.DuplicateAccount when result.Account is not null:
+				Console.Error.WriteLine(Mkb79AuthImporter.DuplicateMessage(result));
 				Environment.ExitCode = (int)ExitCode.RunTimeError;
 				return;
 			case Mkb79ImportOutcome.Success when result.Account is { } account:
diff --git a/Source/LibationUiBase/MarketplacesUi.cs b/Source/LibationUiBase/MarketplacesUi.cs
new file mode 100644
index 00000000..320ec096
--- /dev/null
+++ b/Source/LibationUiBase/MarketplacesUi.cs
@@ -0,0 +1,107 @@
+using AudibleUtilities;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace LibationUiBase;
+
+/// <summary>
+/// Shared copy for the marketplaces feature, so Classic and Chardonnay say the same thing about the same
+/// state. The wording assumes the reader does not know a marketplace other than their own can hold titles -
+/// that is the whole reason they are here.
+/// </summary>
+public static class MarketplacesUi
+{
+	public const string DialogTitle = "Marketplaces";
+
+	public const string Intro
+		= "Audible keeps a separate library for each marketplace. A title bought while your Amazon address was "
+		+ "set to another country stays in that country's library, and Libation only sees the marketplaces "
+		+ "listed here.\r\n\r\nChecking one adds it to this account's library scans. No second login is needed - "
+		+ "your existing credentials work in every marketplace.";
+
+	public const string CheckButton = "Check other marketplaces";
+
+	public const string Checking = "Checking marketplaces...";
+
+	public const string NotAuthenticatedToolTip
+		= "Scan this account's library first, so Libation has credentials to check the other marketplaces with.";
+
+	public const string ButtonToolTip
+		= "See whether this account holds titles in other Audible marketplaces.";
+
+	public const string NoneFound
+		= "No other marketplace holds titles for this account.";
+
+	public const string NoneChecked
+		= "No marketplace could be checked. Scan this account's library to refresh its credentials, then try again.";
+
+	/// <summary>Label for the accounts grid button: the account's marketplaces, at a glance.</summary>
+	public static string ButtonText(int marketplaceCount)
+		=> marketplaceCount > 1 ? $"{DialogTitle} ({marketplaceCount})" : DialogTitle;
+
+	/// <summary>One probed marketplace, as a line in the list.</summary>
+	public static string ResultText(MarketplaceProbeResult result)
+		=> result.Outcome switch
+		{
+			MarketplaceProbeOutcome.AlreadyScanned
+				=> $"{result.Locale.Name} - already scanned by this account",
+			MarketplaceProbeOutcome.ScannedByAnotherAccount
+				=> $"{result.Locale.Name} - already scanned by {result.ClaimedBy}",
+			MarketplaceProbeOutcome.TitlesFound when result.TitleCount == 1
+				=> $"{result.Locale.Name} - 1 title",
+			MarketplaceProbeOutcome.TitlesFound
+				=> $"{result.Locale.Name} - {result.TitleCount} titles",
+			MarketplaceProbeOutcome.Empty when result.TitleCount == 0
+				=> $"{result.Locale.Name} - no titles",
+			MarketplaceProbeOutcome.Empty
+				=> $"{result.Locale.Name} - reachable, title count unknown",
+			_ => $"{result.Locale.Name} - could not be checked ({result.Error})"
+		};
+
+	/// <summary>
+	/// How an account's marketplaces read in a list of accounts. The scan picker in particular has to show
+	/// these: one checkbox there can scan several marketplaces, which would otherwise be invisible.
+	/// </summary>
+	public static string MarketplacesSuffix(Account account)
+	{
+		var extras = account.AdditionalLocales;
+		if (extras.Count == 0)
+			return account.Locale?.Name ?? "";
+
+		return string.Join(", ", new[] { account.Locale?.Name ?? "" }.Concat(extras.Select(l => l.Name)));
+	}
+
+	/// <summary>The row text used by both frontends' scan pickers.</summary>
+	public static string ScanPickerText(Account account)
+		=> $"{account.AccountName} ({account.AccountId} - {MarketplacesSuffix(account)})";
+
+	/// <summary>
+	/// What a probe turned up. A marketplace that could not be reached says nothing about what is in it, so
+	/// failures are never folded into "nothing found" - reporting an unasked marketplace as empty is the exact
+	/// silence this feature exists to break.
+	/// </summary>
+	public static string Summary(IEnumerable<MarketplaceProbeResult> results)
+	{
+		var all = results.ToList();
+
+		var found = all.Where(r => r.Outcome is MarketplaceProbeOutcome.TitlesFound).ToList();
+		var failed = all.Count(r => r.Outcome is MarketplaceProbeOutcome.Failed);
+		var answered = all.Count(r => r.Outcome is MarketplaceProbeOutcome.TitlesFound or MarketplaceProbeOutcome.Empty);
+
+		var unchecked_ = failed == 0
+			? ""
+			: $"\r\n\r\n{failed} marketplace{(failed == 1 ? "" : "s")} could not be checked, so what they hold is unknown.";
+
+		if (found.Count == 0)
+			return answered == 0
+				? NoneChecked
+				: NoneFound + unchecked_;
+
+		var list = string.Join(", ", found.Select(r => $"{r.Locale.Name} ({r.TitleCount})"));
+		var lead = found.Count == 1
+			? $"Found titles in another marketplace: {list}.\r\n\r\nCheck it to include it in library scans."
+			: $"Found titles in {found.Count} other marketplaces: {list}.\r\n\r\nCheck the ones to include in library scans.";
+
+		return lead + unchecked_;
+	}
+}
diff --git a/Source/LibationWinForms/Dialogs/AccountsDialog.cs b/Source/LibationWinForms/Dialogs/AccountsDialog.cs
index e1ad0748..727a5f75 100644
--- a/Source/LibationWinForms/Dialogs/AccountsDialog.cs
+++ b/Source/LibationWinForms/Dialogs/AccountsDialog.cs
@@ -337,9 +337,9 @@ public partial class AccountsDialog : Form
 				return;
 			}
 
-			if (importResult.Outcome is Mkb79ImportOutcome.DuplicateAccount && importResult.Account is { } dup)
+			if (importResult.Outcome is Mkb79ImportOutcome.DuplicateAccount && importResult.Account is not null)
 			{
-				MessageBox.Show(this, $"An account with that account id and country already exists.\r\n\r\nAccount ID: {dup.AccountId}\r\nCountry: {dup.Locale?.Name}", "Cannot Add Duplicate Account");
+				MessageBox.Show(this, Mkb79AuthImporter.DuplicateMessage(importResult), "Cannot Add Duplicate Account");
 				return;
 			}
 
diff --git a/Source/_Tests/AudibleUtilities.Tests/Mkb79AuthExportTests.cs b/Source/_Tests/AudibleUtilities.Tests/Mkb79AuthExportTests.cs
index 1ad5491b..b89b9891 100644
--- a/Source/_Tests/AudibleUtilities.Tests/Mkb79AuthExportTests.cs
+++ b/Source/_Tests/AudibleUtilities.Tests/Mkb79AuthExportTests.cs
@@ -1,4 +1,6 @@
 using AssertionHelper;
+using AudibleApi;
+using AudibleApi.Authorization;
 using AudibleApi.Cryptography;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Newtonsoft.Json.Linq;
@@ -65,4 +67,28 @@ public class Mkb79AuthExportTests
 		auth.BeNotNull();
 		auth.ToJson().Should().Be(Serialize.ToJson(auth));
 	}
+
+	/// <summary>
+	/// The file carries one marketplace because that is all the format has room for: one locale_code beside one
+	/// device registration. Exporting an account that reads several must still name the one it is registered
+	/// with, so the file stays valid for audible-cli - which switches marketplaces from those same tokens anyway.
+	/// </summary>
+	[TestMethod]
+	public void export_names_the_registered_marketplace_and_leaves_the_additional_ones_out()
+	{
+		var account = new Account("user@example.com")
+		{
+			IdentityTokens = new Identity(Localization.Get("ca"))
+		};
+		account.AddMarketplace("us");
+
+		var jo = JObject.Parse(Mkb79Auth.FromAccount(account).ToJson());
+
+		jo["locale_code"]!.Value<string>().Should().Be("ca");
+		jo["with_username"]!.Value<bool>().Should().BeFalse();
+
+		// nothing in the format records the extra marketplace, so a re-import will not restore it
+		jo.ContainsKey("AdditionalLocaleNames").Should().BeFalse();
+		jo.Properties().Select(p => p.Name).Contains("additional_locale_codes").Should().BeFalse();
+	}
 }
diff --git a/Source/_Tests/LibationUiBase.Tests/MarketplacesUiTests.cs b/Source/_Tests/LibationUiBase.Tests/MarketplacesUiTests.cs
new file mode 100644
index 00000000..ff7af45e
--- /dev/null
+++ b/Source/_Tests/LibationUiBase.Tests/MarketplacesUiTests.cs
@@ -0,0 +1,123 @@
+using AudibleApi;
+using AudibleApi.Authorization;
+using AudibleUtilities;
+using LibationUiBase;
+
+namespace LibationUiBase.Tests;
+
+/// <summary>
+/// The marketplaces summary reports on marketplaces that answered. A marketplace that could not be reached
+/// says nothing about what is in it, and calling it empty would recreate the very silence the feature exists
+/// to break - titles present, and no sign of them anywhere in the app.
+/// </summary>
+[TestClass]
+public class MarketplacesUiTests
+{
+	private static Locale locale(string name) => Localization.Get(name);
+
+	private static MarketplaceProbeResult found(string name, int count)
+		=> new(locale(name), MarketplaceProbeOutcome.TitlesFound, count);
+
+	private static MarketplaceProbeResult empty(string name)
+		=> new(locale(name), MarketplaceProbeOutcome.Empty, 0);
+
+	private static MarketplaceProbeResult failed(string name)
+		=> new(locale(name), MarketplaceProbeOutcome.Failed, Error: "Request could not be authenticated");
+
+	[TestMethod]
+	public void nothing_checked_does_not_claim_there_is_nothing_to_find()
+	{
+		var summary = MarketplacesUi.Summary([failed("canada"), failed("uk")]);
+
+		Assert.AreEqual(MarketplacesUi.NoneChecked, summary);
+		Assert.IsFalse(summary.Contains("No other marketplace holds titles"));
+	}
+
+	[TestMethod]
+	public void every_marketplace_answering_empty_is_a_real_answer()
+		=> Assert.AreEqual(MarketplacesUi.NoneFound, MarketplacesUi.Summary([empty("canada"), empty("uk")]));
+
+	[TestMethod]
+	public void a_marketplace_that_could_not_be_checked_is_called_out_alongside_the_ones_that_could()
+	{
+		var summary = MarketplacesUi.Summary([empty("canada"), failed("uk")]);
+
+		StringAssert.Contains(summary, MarketplacesUi.NoneFound);
+		StringAssert.Contains(summary, "1 marketplace could not be checked");
+	}
+
+	[TestMethod]
+	public void found_titles_are_named_with_their_counts()
+	{
+		var summary = MarketplacesUi.Summary([found("us", 50), empty("uk")]);
+
+		StringAssert.Contains(summary, "us (50)");
+		StringAssert.Contains(summary, "another marketplace");
+	}
+
+	[TestMethod]
+	public void several_finds_are_counted()
+	{
+		var summary = MarketplacesUi.Summary([found("us", 50), found("uk", 3)]);
+
+		StringAssert.Contains(summary, "2 other marketplaces");
+		StringAssert.Contains(summary, "us (50)");
+		StringAssert.Contains(summary, "uk (3)");
+	}
+
+	[TestMethod]
+	public void a_find_still_reports_what_could_not_be_checked()
+	{
+		var summary = MarketplacesUi.Summary([found("us", 50), failed("uk"), failed("japan")]);
+
+		StringAssert.Contains(summary, "us (50)");
+		StringAssert.Contains(summary, "2 marketplaces could not be checked");
+	}
+
+	[TestMethod]
+	public void one_title_is_not_reported_as_titles()
+		=> StringAssert.Contains(
+			MarketplacesUi.ResultText(found("us", 1)),
+			"1 title");
+
+	[TestMethod]
+	public void a_reachable_marketplace_with_no_count_is_not_called_empty()
+		=> StringAssert.Contains(
+			MarketplacesUi.ResultText(new MarketplaceProbeResult(locale("us"), MarketplaceProbeOutcome.Empty)),
+			"title count unknown");
+
+	[TestMethod]
+	public void the_scan_picker_names_every_marketplace_an_account_reads()
+	{
+		var account = new Account("user@example.com")
+		{
+			AccountName = "Mine",
+			IdentityTokens = new Identity(Localization.Get("ca"))
+		};
+		account.AddMarketplace("us");
+
+		var text = MarketplacesUi.ScanPickerText(account);
+
+		StringAssert.Contains(text, "canada");
+		StringAssert.Contains(text, "us");
+	}
+
+	[TestMethod]
+	public void the_scan_picker_reads_as_it_always_did_for_a_single_marketplace_account()
+	{
+		var account = new Account("user@example.com")
+		{
+			AccountName = "Mine",
+			IdentityTokens = new Identity(Localization.Get("ca"))
+		};
+
+		Assert.AreEqual("Mine (user@example.com - canada)", MarketplacesUi.ScanPickerText(account));
+	}
+
+	[TestMethod]
+	public void the_accounts_grid_button_counts_marketplaces_only_when_there_is_more_than_one()
+	{
+		Assert.AreEqual("Marketplaces", MarketplacesUi.ButtonText(1));
+		Assert.AreEqual("Marketplaces (3)", MarketplacesUi.ButtonText(3));
+	}
+}