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.
This won’t help, but I keep photos of everything I sell, as I pack it, and the packing. Sorry for your difficulty and suspect you’ll lose the dispute. 
The only way to handle this type of case is to immediately offer a full refund and require the buyer to return the item, it’s the old “full refund if not entirely satisfied” trick and eating the return shipping is much less hassle then dealing with a pita buyer and PayPal.  But sorry you have to deal with this. 
I figured the same but just don’t think it is right to let cheaters take advantage easily. There should be a counter force or what....
You have the option of providing negative feedback to the buyer as well.
Well, how about outing the guy, here?

Maybe a hungry crap eating dog will wander his way....
I had left him a positive feedback after received the payment and shipped the gear out. Now I don’t think A’gon let me change it or leave another. 
I am not sure it is allowed to show the bad guy’s name here. But whoever wants to know can check my profile to find out. Thanks. 
I had a buyer with good feedback claim a woofer was blown on a pair of Harbeth SHL5’s. The speakers were perfect when I shipped them and my buyers claim seemed fishy, so I refunded his purchase and had him ship the speakers to the US Distributor (Fidelis AV) for inspection. Fidelis said the speakers were fine, which confirmed my belief that my buyer just wanted to test drive the speakers. With email proof from Fidelis that the speakers were operating properly, I told the buyer he either had to take the speakers back or I would leave him negative feedback. He quietly agreed to buy them back when Fidelis support allowed me to call his bluff. What a dishonest Jack a--!
Sounds like a crap eating dog would never starve around here. Too bad the op’s Buyer’s full name doesn’t show on his rating. 
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. 
I understood it would be less costly if I offered him a full refund and cover shipping. But he did not seem to really want to return the already very bargain buy.  All he wanted is further discount by threatening me with negative feedback. He did not even prove the claimed gash existed as mater of fact. By the way, the front panel is a big chunk of aluminum plate, I just cannot imagine what could possibly cause a big gash if not intentionally do it.  
Post removed 
Just an update:  
A’gon stepped in to point out he is lying on the power cord case and requested him provide “big gash” pictures. Then this guy is resorting to name calling and ldisrespectful comments on A’gon investigator.  He still does not provide any photos of the claimed damage.  What a irrational A*hole!  
Here are the pictures. There does appear to be a scratch by the display. Maybe an error while changing the display you did not see?
No, that is not a scratch. It is a factory designed “window” maybe for receiving remote signals.  You can check online pictures. Some model has a dot, some has a line/gasp like mine.  
Sorry the above link was not properly posted.  Just google it or go to to see it all.  Thanks. 
No problem at all, thanks for pointing it out. Now I am 100% sure there is no damage on my unit. 
Audiogon will protect you as soon as you are honest. Same will do for the buyer.
Proof what you say to them and they will remove the negative feedback from your file. 
After seeing the way Sarah of A’gon has been handling my case in the last two days, I am regaining my confidence on this community. 
The buyer finally dropped the dispute this morning.  A’gon also helped to remove the false negative feedback on me. Thanks to Sarah @ A’gon!  
Everybody loves a happy ending.  Good to read this.  Kudos to the A'gon staff.

That's great to hear. There's nothing worse than a bad transaction.
Out of curiosity, is the buyer keeping the unit or sending it back?
So far he is not requiring return through PayPal although he threatened to do so. He got a great deal and he knows it. A’gon said it is out of their control if he requested refund through PayPal, but would suspend his membership due to all his claims are fake.  I just don’t understand why people could be so ignorant/irrational and went so low.  Even after the A’gon staff called out his lie on the power cord issue after saw our conversation message recorded on A’gon system, he got so mad and still insisted I must give him a good factory cord.  In regards to the “damage”, it does not exist at all. Under such plain and obvious facts, he still denies his wrong doings and even cursed A’gon staff with disrespectful language. Sellers really need to watch out this person. 
If he drags you into PayPal be Very specific in drafting your reply and they love photos. If you need help just ask. I may be retired but I’m not dead. 🙀
Thank you very much for your support, really appreciated!  I hope he is not that insane.  I am glad and humbled that justice finally served in our A’gon community.  Thank you all and happy holidays!