I think you got a good deal here and, based on your homework, this seems like a legitimate transaction. I'm going to have to bookmark OfferUp.
For what it's worth, since there was some debate on the legality of unknowingly possessing stolen property here, I asked my brother about it since he's a state trooper on the investigations side for Arzizona DPS (Department of Public Safety). He said the answer always depends on the state you're in, and it's not handled uniformly from state to state, with some being stricter than others (i.e., in some states "good faith" defenses may and often do work better than in others). The AZ judicial system apparently uses a quasi-nebulous "reasonable person standard" to determine whether or not the purchaser of stolen property should be charged with possession of stolen property (often felonious). He said if it's too good of a bargain to be true, it probably really is (OP originally went with his gut, which was a good initial response), and it's to your benefit to do everything you can to ensure that the items are not stolen.