It doesn't get much easier than this. A writer over at NeoCrisis was visiting his local Wal-Mart when he saw a new vending machine that allows you to trade in used PS2, PS3, Xbox, Xbox 360, and Wii games for credit.
Wal-Mart has already commented on the new kiosks (which I'm assuming haven't gone nationwide yet), saying that if the pilot program is a success, they'll start providing actual Wal-Mart credits instead of just a charge back. I don't know about you, but I love kiosks, especially when they function properly . . . quick and easy!