I’m a bit late to the party on this one, but better late than never. Online game rental service Gamefly has a service in beta which allows you to trade in your used games for credit. This is an interesting development, to say the least. I have been a Gamefly subscriber almost since the inception of the service, and this is the most interesting development I can remember. The service claims to be in beta, so that implies that not every member will see the new TRADE-IN tab.
The service has some strange terms, though, like the one that says trade-in credit can only be applied to your monthly membership fees, which implies that you won’t be able to use your game credits to purchase used games from the store. Another one says you have to pay for shipping, including on anything they don’t accept and send back to you. Those type of restrictions make the service less valuable, but hopefully GF will see the error of their ways.
Essentially, you just enter the UPC code from the game you want to trade into a form on the Gamefly website. Gamefly will tell you how much it is worth in store credit.  You can add multiple games to your trade-in cart before checking out. Once you do, you just print out the packing slip and ship off your games (at your expense). Gamefly will inspect your order and credit your account at your regular billing cycle.