Thats a good one.
I was looking at an Austrian M95 carbine once, and the dealer yelled "hey kid, get away from that, it is a rare collectors rifle" (I was 27 years old)
I went back to the table that had a badly rusted leg, gathered up as much rust as I could, went back to his table, asked really humbly to look at the rifle again, I tilted the rifle up (barrel down) and dropped the pile of rust to make it look like it came out of the rifle. I made a HUGE deal about him trying to rip everyone off with rusty rifles.
He was so pissed he tried to have me thrown out of the show. It was so funny, the more he screamed, the more people left. I don't think he sold another gun all day.
The next day I told him that if he wouldn't have been an ass I wouldn't have ruined his weekend. He has been a very nice dealer ever since then.