It might be possible that the Wii is more sensitive to scratches than the GameCube, and it's also possible that the Wii isn't reading discs as well as it used to. It's hard to say. But given the appearance of the disc and the fact that the Wii can play other games, I'd say it fair grounds for a return/exchange. But if you're outside of their time frames, you may be out of luck. I suppose it's worth contacting customer support and explaining the situation. Maybe they'll make an exception.

Buying used games from GameStop is really a crapshoot. It's safer to assume you'll get loose copies and just be happily surprised if you receive any packaging. Assume the worse with condition too. eBay isn't without its own frustrations, but unless GameStop offers a great deal, I'd always recommend eBay for buying used games over GameStop.com.