Great work. But in your case you were the person who was aggrieved not some bystander who wasn't part of the transaction.
What's being discussed here is whether some know-it-all at a gun show who goes around eavesdropping on complete strangers should take it upon himself to intrude if he overhears something that offends his over-active sense of social justice. It isn't a life and death issue, oppression, serfdom or any other matter more serious than someone who doesn't know what he's doing being out a few hundred bucks on a gun.
The holier-than-thou who are still determined to intervene and intrude based on their own set of "values" should also be the very last people to complain when the government does exactly the same thing for the same reasons to gun owners. Interventionism at gun shows and intervention by government share the same basic desire to impose your values and viewpoints on another person whether they want it or not.
Tis true, but in this situation, I felt it appropriate to warn others so they didn't get scammed by this guy, felt it was similar enough to merit adding it.