Honestly, it has been my experience that gamestop is a bit of a rip. When they first started out with the whole used game thing they wernt so bad. They actually helped out some of the less fortunate gamers by re-selling the used games and equipment at low prices.
If you look at them now they have all but baztardized the concept. My last experience with them was taking a stack of 5 or so games to lessen the impact of the newley released Mercenaries title. Having paid an average of $50/game figured I would be fairly compensated and get a good price. By the time I left the store I was livid. The crooks gave me @ $2/per game and I have regretted it ever since.
As my WaW disk is cracked I have recently taken the time to check them out. They want $54 for a used copy of WaW!!!! Assumeing some poor kid got ripped for a paultry $2 for that title, they now stand to make a sizable profit. Thats what its all about to them anymore, they will not care that you have taken care of you stuff, they will give you a pittance for your stuff and turn it into thier personal gain.
Sorry for the text wall but those people set me off....
I would suggest e-bay or craigslist....