I go to car boot sales and see a lot of old pictures in nice frames. If I bought a picture and found it was worth a lot of money, would the seller have any claim on it?
No. Once you’ve handed over the money for something it becomes your exclusive property and the seller has no right to a share in the proceeds even if the painting turns out to be a Van Gogh. It does happen from time to time, which is one of the attractions of car boot sales for bargain hunters. If you found the painting in, say, a chest of drawers you bought, it would still belong to the seller unless you could prove he intended to part with it.