Rob raises a good point.
Now, I'm not at ALL advocating that torrenting is acceptable, but...
Someone buys a copy of the game, allegedly, cracks the protection and torrents it. No one but piratebay sees any money from it due to their ad sales.
Someone buys a copy of the game, plays it, sells it to Gamestop (most likely, though, they just traded it in for credit), who then resells the game with them keeping all the money. If you bring the used game back in, the cycle continues.
Now, if you asked someone they would say their Gamestop copy is "legal." My question to developers is: do you consider this copy to be "legal?" My guess is, yes, since it's in its original packaging.