listbox selectedvalue property not giving me string value of selection

Set the ListBox.SelectedValuePath to the Name of the Member in the Binded class that represents the value you need. This way you should be able to retrieve the value via ListBox.SelectedValue

Edit (example):

<ListBox x:Name="TestListBox" ItemsSource="{Binding}" SelectedValuePath="LastName" MouseDoubleClick="TestListBox_MouseDoubleClick">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding Path=FirstName}" Width="110" />
        <TextBlock Text="{Binding Path=Age}"/>
      </StackPanel>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

CodeBehind:

public partial class MainWindow: Window
  {
    public MainWindow( )
    {
      InitializeComponent( );
      var persons = new System.Collections.ObjectModel.ObservableCollection<Person>();
      persons.Add( new Person( ) { FirstName = "Walter" , LastName = "Bishop" , Age = 63 } );
      persons.Add( new Person( ) { FirstName = "Peter" , LastName = "Bishop" , Age = 33 } );
      persons.Add( new Person( ) { FirstName = "Olivia" , LastName = "Dunham" , Age = 33 } );
      TestListBox.DataContext = persons;
    }
    private void TestListBox_MouseDoubleClick( object sender , MouseButtonEventArgs e )
    {
      if ( TestListBox.SelectedItem != null )
      {
        MessageBox.Show( (string)TestListBox.SelectedValue );
      }
    }
  }

  public class Person
  {
    public string FirstName { get; set; }
    public string LastName{get;set;}
    public int Age { get; set; }
  }