Two questions. 1) You list an ordinary 1964 Kennedy half for $15 buy it now on ebay. Somebody buys it. You then notice that because of a typo, somebody bought it for $150. What would you do? 2) You list the same Kennedy half for $15 buy it now. Somebody buys it. You then notice that because of a typo, somebody bought it for 15 cents. What would you do?
In the first case, I'd celebrate. The buyer made an informed purchase and paid what he thought it was worth. In the second place, I'd kick myself and send him the coin.
In either case I'd inform the buyer of their mistake, and offer them a chance to correct it for the proper prices. Most people should understand, and the guy way overpaying will be grateful, and more likely to buy from you again.
I would ask: Who made the typo? Buyer or Seller? If it was the buyer, how did they buy something for more than it was listed?
First Case - because of the typo, no meeting of the minds occurred. Therefore, no contract was formed. Because no contract was formed, there is no contract to enforce. Also, if a contract does exist, it could be invalidated because of mistake or unconscionability. Now, if it were me, I would not take advantage of someone. However, if the coin turned out to be an error or variety, then that would be a different scenario. Second Case - See above. However, I would send the coin to the buyer in the interests of customer relations.
They were listed with BIN prices, not auction prices. Therefore IMHO: The first guy gets a $135 refund if the typo was his, a chance to reconsider if the typo was yours. The second guy's buy doesn't go through if the typo was his and EBay's system is working, so it must have been yours, in which case that's what you offered it for, that's what he agreed to, that's his coin; and you've learned a valuable lesson on the importance of proofreading.
Refund the difference or you're labeled a crook. Ask for buyer to make it right, but I wouldn't hold my breath waiting for them to do so. Next time type the ad while sober so you don't make this mistake again and have to chalk it up to an expensive lesson.
The way the question is phrased, the only one who could have made the typo is the seller. Therefore, the person who bought it knew what he was doing. If the buyer sent me a message saying he wanted to cancel his bid I would allow it, but if he paid me and seemed happy with the transaction, why wouldn't I sell it to him? I've purchased buy it now items that were priced low and then had the seller tell me the transaction was canceled because they screwed up. I didn't like it but I didn't say anything either. However, if I had listed the coin for 15 cents by mistake, I would honor the deal.
