You may be surprised to learn that your old iPhone could be worth anywhere from $50 to over $200 depending on the model and condition. Twice I’ve sold old iPhones – once on eBay and once on Amazon. This article from the New York Times discusses some of your options.
Two very important notes before you sell: reset the phone to factory settings. To reset your iPhone go to Settings -> General -> Reset -> Erase All Content and Settings.
Second, remove and destroy the SIM card.