You have been an exemplary seller. There is nothing wrong with contacting a buyer to see if they are happy. Opens the door to questions they might have about the operation of the unit and exchange of general audiophile chit-chat, which is why we are all here in the first place, is it not?
My sense is that the guy got buyer's remorse. Maybe he saw a deal he liked better somewhere and was gnashing his teeth at jumping at yours too soon. When you contacted him he saw a way out and didn't give two hoots about your frustration. That comment about owning 800s sounds very defensive IMHO.
Either you can just let it go and move on, or you can do something. My sense is that you would rather do something, which is why you posted here. Anybody who criticizes you for that should mind their own business.
I think you should ask the buyer what he is using to make the copies he says are skipping and then see if you can make a few yourself. If the copying software/machine is something you can't get your hands on, maybe somebody in one of these forums can help. Put up a post explaining your position and I am sure you will get an offer of help somewhere. You are just the kind of guy we all want to do business with. If there is still no skipping, contact the buyer, restate your position that the player does not skip and tell him that you would like compensation for the shipping charges. There is no reason you should be out anything but time and frustration just because this guy is behaving like a girl.
As for the bad feedback, don't rely on the opinions you are seeing posted here. Contact audiogon and tell them what is going on. Tell them you feel you are justified in laying bad feedback on this guy, but that you are worried he will retaliate. This would be a good way to find out if the new feedback system is as worthless as some posters here seem to feel it is.
Good luck.