As I have shared with the forum, some people like to take lunches, I like to look up the local coin shops throughout my travels.
On one afternoon, I had the opportunnity to look at about 6, 1960 "small date" proof sets. One set caught my eye as the Lincoln was a much better red proof than what I had so I bought it. Paid $50 (cash) for the complete set (with original envelope and all).
I didn't think much about it for awhile until one day I decided to upgrade my registry set and buy a 1960 SD over LD Proof
Lincoln Cent. I have been saving for a while and I won one via Heritage Auctions that cost me $1,265 with fees and all. Ouch! Population of 22 with only 6 better can be expensive, but I was in a "got to have it mode" and I took the plunge.
Back to that 1960 small date set. When I got my coin in from Heritage, I really examined it under the loop so I could appreciate the SD over LD. Then I took out my proof set and examined that Lincoln more closely and thought, "Gee...that looks very close to what I just received."
I took it to my local coin guy who concurred, so I submitted it to PCGS. It came back as a PR68RD SD over LD! A
68 RED with a population of 6 with none higher!

My $50 proof set had a $2,400 coin in it! Well, now I have
two 1960 SD over LD Lincolns proofs and I am going to sell the one I just got from Heritage. If anybody is interested, let me know.
So the moral of the story? Check your coins! You never know what you may find. BTW, I didn't have the heart to call that dealer and tell him, but I sure it is not the first time a dealer sold a coin that was worth a bunch more than what it was sold for.