The one that leaps out at me is the Mayo Hugh Duffy. I really liked that card, found one at a very reasonable price, and held it for a long time. Then someone started making offers on it, so I set a price and they paid it. I regretted the sale for years until I was able to buy a replacement several years later.
The other one is a rare Cuban tobacco card from a brand called Turquino. There was one known example of Dempsey and one of Tunney. A big Dempsey collector approached me to buy my Tunney and made a ridiculous offer so I sold it. He quit a year later and I got it back for a lot less.
