<ComboBox ItemsSource="{Binding AllCustomers}" IsEditable="True"/>

<ComboBox ItemsSource="{Binding AllCustomers}" DisplayMemberPath="CustomerName" IsEditable="True"/>

<ComboBox ItemsSource="{Binding AllCustomers}" SelectedValue="CustomerName" IsEditable="True">
                        <ItemsControl.ItemTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding CustomerName}"/>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ComboBox>

public class AllCustomersViewModel
{
   public ObservableCollection<CustomerViewModel> AllCustomers {get; set;}
}

public class CustomerViewModel
{
   public string CustomerName;
   public short CustomerID;
}

public class AccountTransactionsViewModel
{
    DataRepository _repository;

    public AccountTransactionsViewModel()
    {
        _repository = new DataRepository();
        CreateAccountsViewModel();
    }

    public ObservableCollection<AccountViewModel> AllAccounts { get; set; }

    void CreateAccountsViewModel()
    {
        List<AccountViewModel> allAccounts = _repository.GetAccounts()
                                                        .Select(a => new AccountViewModel(a, _repository))
                                                        .ToList();

        AllAccounts =  new ObservableCollection<AccountViewModel>(allAccounts);
    }
}



  public class AccountViewModel
  {
       Account _account;
       DataRepository _repository;

       public AccountViewModel(Account account, DataRepository repository)
       {
            _account = account;
            _repository = repository;
       }

       public short AccountID { get { return _account.AccountID; } set { } }
       public string AccountName { get { return _account.AccountName; } set { } }
  }

<ComboBox Name="customerCombobox" ItemsSource="{Binding AllAccounts}" IsEditable="True">
                        <ItemsControl.ItemTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding AccountName}"/>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ComboBox>

<ComboBox ItemsSource="{Binding AllCustomers}" SelectedValue="CustomerName">
        <ItemsControl.ItemTemplate>
             <DataTemplate>
                 <TextBlock Text="{Binding Item.CustomerName}"/>
             </DataTemplate>
        </ItemsControl.ItemTemplate>
</ComboBox>

<ComboBox Name="customerCombobox" ItemsSource="{Binding AllAccounts}" SelectedValuePath="AccountName">
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding AccountName}"/>
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                </ComboBox>