Sender is not the selected object, you must to get the selected object from listBox directly:
private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
ListBoxItem selected = listBox1.selectedItem as ListBoxItem;
}
or
private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
ListBoxItem selected = listBox1.Items[listBox1.selectedIndex] as ListBoxItem;
}