Not to further the argument or play devil's advocate, but even that only says you just owned it at one time. What if it was sold to a friend and you stole it from him? Fact of the matter is if there's no reason to suspect it's stolen and you're not a prohibited person from owning a firearm, it's legally owned.
Another thing is that when the dealer takes the gun in like that, the dealer needs to complete a RI-60 with himself as the purchaser. If the gun was stolen, I'm sure the person trying to trade it in would not want to go through the process at all.