If you want to save the items to a database, you can take advantage of the Automatic Database Update feature of the RadListBox. You can see how it works here.
Or, if you want a more custom scenario, you can use the Transferred server-side event to get the transferred items, and store them in a database, or implement some additional logic.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed