I can attest to coin shops doing this. I had one LCS give me a couple of loose proof Kennedys in change, and another time I got a proof Ike dollar. I had another shop hand me change in worn silver certificates and red seal $2 notes. I would imagine a lot of this stuff comes in with random collections and it's not worth hanging onto, but it's fun for customers.
Things like this have been going on for a long time. There is a point where dealers have to much stuff that won't sell. Coins can be brought in from an estate but, the person that brought them in thought it would be a good idea to polish the coins up before trying to sell them. Horrifying but, happens.
There is another reason some coins show up in the market. During the great depression a lot of people were forced to spend some old coins for food. I have another theory that I think is valid. I have noticed over years that there are a lot of really high end coins that show up on the market from Germany. Most date From around 1900 to the early 1930's. My theory has two different scenarios and neither is good. Most collector are aware that after WW 1 the value of German money was almost zero. You can look this up if you don't know. In this situation where your family is starving I think a lot of collector coins were traded for food. If your kids are starving you will do what ever you can to feed you kids. If you had silver or gold coins you could buy a lot of food. The second situation is far worse. When Hitler came to power many Jewish people knew their days were numbered and those that could left the country. However, as it became more evident that life as a Jew in Germany meant you were going to die. Not all realized this but many did. So I believe between 1935 and 1939 many of these high grade German coins were used to bribe their way out of the country. These coins could have also been taken as war booty by the invading armies. It wasn't just the Jews that were in danger but, other people were on the hit list. Russian, Americans and other invading armies took war booty. This is a know fact. I also know the Germans and Russians did the same thing in Poland and the Ukraine. Personal family knowledge. My grandfather's family that lived in Poland at the time were taken to German slave labor camps. Most didn't survive. My grandfather's brother was shot by the Russians. What I've written is only my theory.
I think he was referring to the fact that folks went off on a tangent over their ancient coins instead of staying on topic...no biggie, everyone is proud of their own collections and we all love to share!
I bought an old coin book from Lincoln Coins in Buffalo. Was 50 cents. Gave dealer a buck. He gave me a proof Kennedy in change. I left a 5 on the counter as I left