Difficult buyer

[harm_less]

Just sold a car part, they asked if they could offer me $500 which i accepted, now they say they found the same part at a wreckers for hallf the price and asking if I would match it?
Should I just walk away from the trade? Ive told them that this part has been refirbushed with a new motor and that I doubt the used part at the wreckers has had this done, I also explained the costs involved.
Why bother bidding?

Walk away, they're playing games. If they get pissy about it blacklist them. If "accepting the offer" is by doing Buy Now contact TM for a success fee refund.

I assume this discussion is in the listing's Q&A so your defence by telling them the part is of improved condition may well coax other buyers to get on board.