um...I thought Gamestop's website gave their trade-in values themselves. You do realize that trading in games is a MAJOR rip-off, right? As in simply giving them your money for buying a brand new game automatically degrades their value for the game at least $20 (seriously, even if its unopened, ever, they'll still majorly drop down the accepted value. You don't even have to leave the store)
While I agree that they will rip you off whatever you do (they are aiming to make a profit, and as big as possible), if you are going to trade games in (especially if you never play them and need the cash, you might as well), i'm pretty sure Gamestop will give you their values for major games on the website, although you can probably make more selling them through Ebay, etc.
They're doing a 50% trade-in thing right now on games, not sure how far that applies but the potential fraud of the ad makes it look like any modern system. That would be probably worth it, but ebay or even opening an amazon account would be smarter.
And if you think they give the best, go find a ma and pa type private shop as they know if a game is worth something and will pay more in credit or cash than GS as they're the lowest of the low.
That +50% added trade in value at Lamestop ends on close of March 17th in the states. If they didn't intentionally lower their values due to this deal now is the time to strike. A $60 game could get you usually $30 but in this case $45 in trade-in right now which is near enough to be fair.