Yeah, it does say one coupon per visit, so there really isn't anything stopping you from using it at the same place multiple times. I guess, if they wanted to be huge dicks, they could bitch at you and say you're a "dealer," but I don't see that happening. As the above poster says, they want your business, especially with something like this. We're doing them a service, taking a load of old games off their hands. Hell, they are probably still making a profit, considering the crappy trade-in values they have.
By the way, has anyone else noticed these trends at the stores they have visited? I've gone to three, and the GameCube games are beat to hell and generally only consist of multiple copies of Zelda and Mario games. I have only seen a handful of anything else, especialy third-party titles, and certainly nothing worth getting.