I think the main reason they allow buy backs is because it's just easier than trying to figure out who's zoomin who. You'd never completely stop it, operators are shady and would just bid on their friends or whatever. Also if you think about it, they don't really charge a buyback fee, they just charge the seller the same fee they charge him if he buys anything.

So basically they're just keeping it super simple instead of trying to police every single item to make sure the owner or one of his cohorts isn't bidding on it.