One tip off is they almost always work in pairs when trying to get someone to screw up. One does the talking and the other is the silent buddy observing everything.
Never seen them at a table, but have suspected the above to be true - you know, those "deals" that seem too good to be true. The random, shady looking guy who has a van out in the lot with stuff that you'll just love
Happened a couple times now - may
have been legit sellers, but I always trust my gut, and my gut said run