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It works for sure in the 2ds. Not sure about the 3ds, but I am pretty sure it does.
Generally, GameStop's games are in fairly good condition (there may be some scratches and damage to the game card/label) and they work, although sometimes the contacts may need to be cleaned first. Once in a great while, you may get a game that doesn't work, but, as long as you keep your receipt, you can return it for a refund or exchange. I bought a copy of Style Savvy for the Nintendo DS from GameStop.com some time ago and it worked fine for me with no problems.