private void button1_Click(object sender,EventArgs e) 
{ 
    for(int x = listBox1.SelectedIndices.Count - 1; x>= 0; x--)
    { 
        int idx = listBox1.SelectedIndices[x];
        listBox2.Items.Add(listBox1.Items[idx]); 
        listBox1.Items.RemoveAt(idx);
    } 
}