IMHO Nathan there will be very little you can do about it. If you speak to a CSR I'm quite sure they will just say that is the buyer's opinion of the transaction and the feedback will stand. Very difficult to get them to remove feedback unless it clearly breaches policy and even the it's difficult.
I would probably just leave a factual and unemotional reply and then put it behind you and move on. Make itclear to iother readers (buyers) what has happened and it should not affect your selling.