Ever heard of Playerchain.com? Check it out. You can list any games you want for what price you think you can get for them. You "trade" the game for Game Bucks and then can use them later to purchase a game from someone else. A better deal then a used game store because they give you MAYBE 25% of the value of your game? They have to so they can turn a profit on it.
For example, I "sold" Black & White 2, Soul Calibur 2 (GC), SSMB, Olivion CE (PC DVD), and Chromehounds. In return, I "bought" Dead Rising and Splinter Cell Double Agent and I still have $32 Game Bucks sitting in my account to use. It only cost me $5 to Playerchain ($2.50 per game received) plus shipping the games I sold.