How to protect seller's rights?


The A'gon's feedback system is lack of seller's protection in my opinion.   Recently I sold a PS Audio P10 to a guy.  I had clearly told the buyer in communication messages (before he committed to buy) that this unit does not come with a factory power cord, and he acknowledged.  But after he received the unit yesterday, he still fired all over me by claiming that I promised to provide a decent factory cord.  I asked him go back to the messages we had before the purchase, and he simply ignored.  He also claimed that there is a big gash on the face plate which I "intentionally" did not disclose.  I have to say that I am not stupid to do such low class disgraceful thing as a long time audiophile.  Nevertheless, there is just NO WAY for me to prove that he is lying.  If I requested a picture of the damage, he could easily make the damage and insists that I did it.  Then he threatened me  with 24 hours to come up a solution to make him happy.  Guess everybody knows what he meant.  I was pissed off  and refused to give in.  Then he left a negative feedback on me today and said he will ask PayPal for a full refund.   I filed a dispute to A'gon this afternoon, but honestly I doubt how much they can do to help me.  I don't think there is a way to simply cancel my already left positive feedback to this dishonest buyer, or, simply tell my story after his false accusation feedback.  Please pitch in to help protecting A'gon sellers, thank you.
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This buyer has a 48 positive feedback which gained my trust. He tried to lowball me first, and I rejected his offer.  We contacted again later, and he mentioned he just bought a Synergistic Research unit. A few days later, he came back to me and said the Synergistic Research unit he bought turned out to be damaged hence he was requesting a full refund and therefore would buy mine again.  I felt a bit fishy but did not put a brake on it. Now it happened to me!   A quick lesson I learnt is never leave a feedback before the buyer left you one. This may not be fair, but what else we can do?  We sellers are paying all the fees and deserved better protection, do we?
As of this morning, A’gon staff has confirmed with me that the buyer has no ground to complain about the power cord since I had made it clear to him before the purchase.  A’gon is request pictures of the so-called big gash on the face plate from this guy. I wish he would not make up one of his own.  Anyway, at least he should learn a lesson (at my cost) it is not that easy to cheat on other people and walk away to his benefit only.  
Agree with folkfreak. Immediately offer a full refund upon return of item. Yes, probably best to offer return shipping cost too. Don’t get into a complex discussion or a war of words. There’s no "make him happy" endgame for such an unreasonable and aggressive person; offering up the reset button will buy you back your sanity as quickly as possible, and shows you to be an honest seller. Try to avoid a long slog with Paypal and/or Audiogon reconciliation. Most likely the guy got buyer’s remorse and is throwing a toddler tantrum to get his way. Fine - just end it ASAP, and put him on your very own black list for next time!

Hope you can soon get back to enjoying this hobby. Sorry for your troubles!
My policy is refund purchase price to unhappy buyer, after the item is returned, but they pay shipping. I only leave feedback after the buyer has confirmed condition and functionality of the item.

A person who wants to be a painful purchaser can usually accomplish their goal. You are better off moving along and selling to someone else.
Yes,  it just takes one irrational hothead to put a small blemish on hundreds of all positive transactions. Nothing one can do. Been there, experienced that.