[pbspelly]

Someone may already have posted about this, but from a legal perspective (as opposed to an ethical one), a lot depends on (a) the knowledge of the buyer and (b) the reason that it was underpriced (i.e whether it was a clerical error or a misunderstanding about the value of the card). The following is copied over from an online legal site (and is thus not intended as legal advice from me):

Unilateral Mistake

A unilateral mistake is a mechanical error of calculation or perception concerning a basic assumption on which the contract is formed. For example:

The Boston Red Sox and Ramon Garcia orally negotiate a contract where Garcia will play for the Red Sox and the Red Sox will pay him $15,000. During the negotiation, Garcia thought he heard the Red Sox say $50,000. This is a unilateral mistake.

The general rule involving unilateral mistakes is that, if the non-mistaken party either knew or should have known of the other party’s mistake, the mistake is a “palpable unilateral mistake” which makes the contract voidable by the mistaken party. For example:

The Pentagon is accepting bids from ship building companies to build a new aircraft carrier. Ten different companies submit bids. Nine of those bids range in price from $140 million to $150 million. The tenth bid, belonging to Seven Seas Shipbuilding Inc., comes in at $43 million. The Pentagon quickly signs Seven Seas to the contract. The next day, Seven Seas reviews its bid submission and discovers some calculating errors that resulted in their bid being $43 million when it should have been $136 million. In this case, the contract will be voidable by Seven Seas. The fact that there was a $97 million difference between Seven Seas’ bid and the next lowest bid should have been a clear indication to the Pentagon that Seven Seas had made a mistake somewhere. Therefore, the Pentagon either knew or should have known that Seven Seas made a mistake. That being the case, the mistake was a palpable unilateral mistake and Seven Seas can void the contract. See M.F. Kemper Construction Co. v. City of Los Angeles, 37 Cal.2d 696 (1951).

Please note that palpable unilateral mistakes will only make a contract voidable if the mistake is a mechanical error (ex: mistakes in calculation or perception). Mistakes in judgment as to the value or quality of an object will not make the contract voidable. For example:

George is the owner and manager of Babe’s Baseball Memorabilia. Mickey is rummaging through his attic one day when he finds a baseball bat signed by Ted Williams. Mickey, who is not a sports fan, has no idea who Ted Williams is but he remembers that there is a baseball memorabilia shop a few blocks away that buys things with signatures on them. Mickey brings the bat to George who offers Mickey $200 for it. Mickey gladly contracts with George to sell the bat for $200. A few weeks later, Mickey is telling Roger, an avid sports fan, about the bat and Roger informs Mickey that the bat was worth $5000. Unfortunately for Mickey, the contract he made with George is enforceable because Mickey’s mistake was not a palpable unilateral mistake. It was simply a mistake in judgment as to the value of the bat.

If the non-mistaken party either did not know, or had no reason to know, of the other party’s mistake, there is a binding contract.

Several modern cases, however, have determined that if the mistaken party notifies the other party of the mistake before the non-mistaken party relies on the mistake, the mistaken party can rescind the contract.