I tried to sell an old (age-wise) gun to a pawn shop, and they refused to buy it because it didn't have a Serial Number. It was too old to need one, but they said that they check all the SNs to make sure they're not stolen and they wouldn't buy it. So, I definitely trust Gun Shops and Pawn Shops. But, I kept the documentation, in case the lack of a SN was questioned. As previously said, keep the documents. One dealer suggested I carry a photocopy, and keep the original in a safe place. I doubt very much that stolen guns would show up on Auction Sites, such as Gun Broker and Auction Arms. Since, these sites are probably checked by ATF and FBI. I have had the chance to buy from persons I know aren't trustworthy, and I didn't because of the risk of it being stolen. One was known to be a gang member and had been arrrested several times, including Burglary. That is a serious red flag. I wouldn't buy from a person I didn't trust. Keep that in mind.