and into a blank listbox...?

[HumanCompiler]

Just make sure you have .EnableViewState set to true for both of your listboxes, then all you have to do is something like this in a button for transferring one item to the other listbox

ListBoxNew.Items.Add(ListBoxMain.Items(ListBoxMain.SelectedIndex))

Enabling the ViewState, will make it so the new listbox retains its list on each postback