Well way back in the 80's I bought 12 $10 gold coins, 1/10 oz each, for $25 each, sold em a month ago for $170 each.
Bought a SW Model 29 for $300. sold last year $1,000
Mosin M27 for $30.00. The rifle was sitting in an old barrel in the corner, no price on it. I told the guy there, I just had $30.00 on me and he said take it
Walked in the owner said "you like those dang military guns what is this" told him it was a MAS 36. He said give me $25 and it is your. Turned out to be a G serial number, made in 1937 all matching, never rearrsaneled. No import marks.
Most recent was a Voere Titan II for $400. They had never heard of them.
My best find is not actually firearms related - last year I went to North Georgia just to do a road trip on a Saturday. I found an older Marine Corps Training Manual from the 60s in there for $5.00 in a pawn shop someplace near Hiawassee - well happens that when I flipped thru the first 100 pages I found $350.00