How I see this...
If the buyer honestly didn't know the value of the mask or had a clue that it could be worth extremely more, then they did nothing wrong. However, if it were me, I'd gladly put some money back into the pockets of the original person that sold it to me. There'd be no obligation, but in my mind, it's karma.
If the buyer did know/understand that it could be worth much more, then ethically could have considered this in the transaction. If they didn't and just wanted to make a killing, then they're probably no different than a dealer of any other collectible who is an a-hole shyster.
Now apply this to the flea market - people go in search of such items at a flea market. What happens if the item is a fake, then the person that bought it thinking it was worth A LOT MORE (but probably didn't disclose that to the seller) got ripped off whilst thinking they were going to make a killing on it - that's just karma... what goes around comes around...
