I'd throw in a 5% restocking fee as well as reimbursement for the fees he paid. Look at it from his standpoint. He put the part up for sale with a time limit on the bidding. Maybe he needed the money to buy something that he wanted and now he has to do what you're doing. It's a vicious cycle. He's out of time as well as money.
Another option, if you can, is to just pay him and then put the part up for bid again to recoup your money for the thing that came up. Do you know who the second highest bidder was? Maybe they'll take it off your hands.
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351W 89 Mustang GT Convertible