GAMESTOP DOES NOT CARE ABOUT SELLING NEW PRODUCT.
They are a pawn shop, and it is best to deal with them as if they are a pawn shop. As confidenceman said, they actively do not want to sell you new product, and vastly prefer when you purchase new product somewhere else. The only exception is when the new product has been preordered, at which point, it will always still be sealed.
Now, massive abuse of the return policy is why there is no longer a 7-day return policy on new games at Gamestop. Don't be a dick by helping to end the return policy on used product as well. Gamestop is a pretty horrible place, and is terrible for the industry in general (hell, piracy is healthier for the industry than used sales are), but is is nice to be able to grab a game, see if I hate it, and return it when I do. It is not nice to be a douche and try to use the return policy as an eternally renewing free rental system.