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.