Unscrupulous ebay buyers

(1) @jl35 has an important point.
Using PayPal as the payment intermediary can shift the discussion away from terms and conditions in your EBay ad , to vagaries in terms and conditions in PayPal. PayPal has its legendary warts and risks causing endless irritation and frustration when it comes to dispute resolution between buyer and seller.

(2) Warranty claims against unit build defects are generally those against the OEM manufacturer’s designated distributor. Intuitively, if the Seller is not an authorized retailer with Distributor, then that warranty option generally is rendered null and void in the fine print templated in 99+% of product sales documentation

(3) Unless otherwise forced upon you by PayPal TBD, the AGON Court of Public Opinion can sympathize with your reported facts situation and your further response options.
My take: Unless otherwise forced into further formal written debate with Buyer, many forum readers would similarly cut him off with the common slang profanity pejorative embodied in Genesis 9:7.

Carry on and good luck.

There are scams galore out there. He might be keeping the good one that you sent and sending back a bad one that he already had.

Thank all for you input! I'll be sticking to my guns, no refund. I'm not sure of warranty, I informed him to contact the manufacturer and take it up with them, as I do not provide a warranty. I clearly indicated this in my listing, no returns, no refunds, all sales final, no warranty expressed or implied. Yet, he continues to message me. I'm not taking back what was a brand new cartridge after he man handled it. He may have caused damage during the install for all I know. People are just so entitled today. Honestly, he just does not like the cartridge and thinks I'm foolish enough to take it back. Anyhow thanks again!

I have sold hundreds of items on eBay and thankfully only had 2 issues - one similar to yours. A guy bought a vintage guitar amp from me which I said worked fine in the description, but that due to the nature of vintage electronics was not warranteed, and no returns are accepted. I even checked off the “no returns” box on the listing form. The buyer obviously decided he didn’t want the amp after all, and said it wasn’t working, filed a defective item claim, and eBay let him return it in spite of my appeal. When I received it, the amp was fine, I sold it again to a satisfied buyer. It cost me shipping both ways, which wasn’t cheap.  I now sell guitar/amps on Reverb and avoid eBay ( primarily due to their excessive fees).

You may wish to read up on eBay's rules, regulations, etc. governing purchases and returns.  As I understand it, eBay is no longer connected with PayPal - they deal directly with the buyer's credit card and the seller's bank account (which, as a seller, you were required to give them access to). This allows eBay to essentially enact a  refund without your consent (they call this "buyer protection.").  Second, if you sell on eBay, there is no such thing as "no returns."  Buyers can initiative the return process at any point and for any reason - even if you've clearly stated a "no return" policy.  All this was designed to eliminate unscrupulous sellers; now it empowers unscrupulous buyers.  In a nutshell: eBay contends they merely provide a platform for buyers and sellers to connect - after which they assume no responsibility.  Since they get a piece of every transaction (regardless of outcome), they're content to simply let buyers and sellers eat one another alive. Financially, it's a foolproof business model; ethically, not so much.  My .02.  If I'm wrong on this, I stand to be corrected.