My wife and I once sold really cheap what we thought were two prints, 1) a Chagal, and 2) a Salvador Dali. Mu mother-in-law, who gave them to us, just informed us that the Salvador Dali was in fact a signed lithograph and is worth between $2000-$2500.
Do we have any recourse to try to get it back from the people who bought it from us (assuming that we can locate them)?
Indeed you do. If it can be proved that you unknowingly undersold for at least a sixth of the price you can invalidate the sale retroactively, returning the money and receiving the print in return.
Although there is a time-limit for claiming Ona’ah, this applies only to the buyer, who has the opportunity to show it to others and discover the real price, and if he does not return within the designated time he forfeits his right to claim Ona’ah. However, the seller, who no longer has the item in his possession, may claim Ona’aheven much later, when he discovers that he has undercharged for the item.