Possession of stolen property is a crime. Yet how can an honest buyer tell if something is stolen?
As mentioned, even a dealer may have some of these by now, and has no way to check them. Lots of used guns on the market on any given day.
If it comes up clean you at least ran the check.
If not, either don't conclude the purchase, or you may well have to surrender the arm as stolen goods and answer some questions about where you got it, which means you are out the purchase price. Beats being charged with receiving stolen goods. Or you can take your chances.
Me, I hate thieves, so i really don't want to reward them. I have been pretty careful about who I buy what from.
In these parts, gun shows are fairly small, and the local police know pretty much everyone with a table (and check out those they don't).