I can top that, I think.
Last June, I went down South to my Great-aunt's funeral. Rented a newish Lincoln, as my old Taurus wouldn't make the trip well, and Mom insisted on something uber-safe.
Went to the graveside service, started back North, and stopped at a decent-sized antique mall on US-250 (Any OH/WV forumites can email me, I know a bunch of good ones in that strip). There was a good Case 6231 there, Tested, great bone, but someone had taken the edge off with a grinder. No big deal, as he only wanted $100 for it. Looked in my LG4 when I went back the car, it was worth about double.
I figure, buy it. I bring the knife up, and do the usual "Take $90?" for it. Guy dang near strokes out on me.

"$90? You're driving a new Lincoln, wearing a BMW hat so you likely own one of those, and you can't give me a measly $10? Not a chance. I'll let it sit before I deal it for $90." I light into the guy a bit, as I've had a rough day- first off, I said, I didn't own the car. Second, I didn't own a BMW at the time, third, he could shove the dang knife, I wouldn't even take it for free. His attitude had spoiled what I thought was a pleasant find.
Guy who ran the booth came up, saw the whole thing, my side and the owner's, and pulled me aside. He told me $80, as that was what he was going to get from the mall as it was- owner guy just wouldn't get his cut. We shook on it, I slipped knife in pocket, smiled at the fuming owner-guy, and walked out.
