So was the x amount more or less than the winning bid.
If less he had a result if he pays and you have if he does not pay as you can give the second highest bidder the opportunity to buy at what would be I am guessing also above your agreed fee.
If the winning bid was less than the agreed price he may be peed off that he could have won it cheeper just by bidding and decided not to buy, in which case you would be on shakey ground I should imagine as eBay tell you not to accept offers outside of the auction.
And if the auction was deliberately manipulated by false bidding by seller or friends of to put up the price , that’s fraud I think iremember someone going to prison for deliberately bidding on their own auction of a minibus to put up the price.