ORIGINAL: gutshot
You may also want to be careful that you're not charged for receiving stolen property. If you go into a store and pay that much less then retail you may not be able to conince the police that you didn't know any better.
If he were the only person who purchased the items, perhaps. But because there were quite a few other customers who also purchased merchandise from the store for the same prices, and because the store owner/employees have already admitted to making a mistake, the police can't do anything about it.
If I were you, I would keep it, no questions asked. You made no mistake, it was the store owners fault.