The law already requires merchants regularly dealing in goods of the kind warrant that such goods be delivered free of any rightful claims by a third party.
You're flipping the burden here--merchants are simply in a better position to ensure the goods that they sell aren't stolen. It's just better policy.
I'm an individual rights/individual responsibility guy. People who want to sue dealers for their poor decisions don't usually post on FR in my experience.
This guy spent more money going hunting in a foreign country than he did on his used Mossy. Next time, he ought to go hunting in the US with a newly US manufactured gun and there won't be any trouble.
Receiving stolen property is receiving stolen property. Let the buyer beware.