Has it been confirmed whether the game does state "Not for resale" on the packaging? Just thinking of possible trade in options that are not GameStop.
They should take it regardless. They're supposed to at least, unless you get a dumb employee (which has happened to me before).
All that the 'not for resale' label means (which is only on the back of games that come in bundles) is that the store cannot take the bundle and separate the items in order to sell them for more. So if a PS3 were bundled with a game for $200, the PS3 costs $250 by itself, and the game costs $60 by itself, the store cannot separate the bundle in order to make $110 more than they would selling the bundle.
It's the same as the 'do not remove this tag' tags on mattresses. It only applies to the retailer that bought the item to sell originally.