I had a few nice finds when I was collecting. One of them was several 5 Franc coins that were issued in 1939-1941 in colonial Algeria. I found them in a street market in Bamako, Mali. They were in a clay bowl and covered with dust, but when I wiped the dust off, they were clearly nearly mint. The vendor haggled and sold all of them to me for a few dollars. I dragged out my World Coins book when I got home and was startled to see that they were valued at about $30-$40 each. I sold most of them on eBay.
Another find was from my next door neighbor in Alaska. I was telling her that the market for silver coins online was pretty brisk and she mentioned that she had some silver coins that she’d like me to sell. So I went over the next day and she handed me three bank bags full of coins, which surprised me. I dumped out the first bag and found it full of 1964 Kennedy silver in nearly mint condition. The second bag was full of silver Franklin half-dollars, in varying conditions, but with some really nice examples. The third bag was full of Liberty half dollars, a lot of which were very worn, but the silver content was there of course. I ended up selling most all of them for her at about $40/roll, and bought some of the better exemplars for my own collection.