There is a website called
Cell4Cash.com that will buy your old cell phones from you. Whenever I switch carriers or phones, I sell my old phone here. You can enter your phone information on the site and they will tell you how much they will pay you for it. If you agree to sell, they send a postage paid box for shipping the phone. In a few weeks you have a check! I just sold an old AT&T phone to them today for $34. Some phones aren't worth that much, but I figure it's better than nothing (since I can't use the phone any longer anyway).