Let it go to EBAY arbitration. IIRC, he can't leave feedback until after EBAY rules. I had the same thing happen to me with a tube a long time ago. I was on your stylus's guys end as the buyer. The tube blew within the first couple hours of a perfectly functioning amp.
I simply opened a complaint with EBAY after trying to work something out with the seller. He was insanely unreasonable and fowl mouthed, etc. Ebay gathered each side of the story as well as the inappropriate and lewd emails he sent me and ruled in my favor. They also erased the negative feedback he left. This was about 6-7 years ago and the feedback system was different.
I guess what I'm saying is for $23 just give in to protect your seller rating or if you feel you have a strong case and things were spelled out very clearly in the auction, take it to EBAY and then abide by what they decide. Based on the facts you've presented I think you have a strong case, but EBAY has shifted to supporting the buyer more than ever, and my experience above is based on the old EBAY where buyer and seller were more or less equal.