I'll recount this without revealing whether I was the buyer or the seller in this scenario so as not to skew reactions. I'm a reasonably experienced ebayer on both sides, as it were, with no previous problems as either a buyer or a seller.
So. The auction in question is for a reasonably heavy, expensive but not particularly rare or unusual piece of equipment, which routinely fetches good prices on ebay, both used and new. The final sale price is about average for that type of item. The winning bidder pays via paypal within the hour. All good. There follows a panicked email in which the buyer explains that they have just realised that the item they bid on (and won) is in fact not the right spec for what they wanted. Buyer apologises profusely for misreading the listing (which was clear), and asks the seller to consider cancelling the transaction and refunding the payment.
Seller emails back saying they will contact the next highest bidder, and cancel the transaction only if the second chance offer is accepted. The following day seller emails the buyer to say that the second chance offer was not accepted, and that the item will be sent as per the terms of the ebay contract.
Clearly neither the buyer nor the seller is contravening the ebay rules here: the buyer made an honest (if dozy) mistake, the seller insists on the transaction being honoured, pointing out that the unwilling buyer can always resell the item. Fair enough.
But what WWYD, as the seller? Make the buyer go through with the transaction, or accept the honest mistake, curse under your breath and relist the item?
[curious]