I got him to confirm the serial number on the lens (which I had photographed) and paid for him (insured) to send it back. I have it now, and it is indeed the same lens he sent me, and it works fine! I guess I basically have to refund him though?!

Please Help.
Thanks !

I didn't find the right solution from the internet.
References:
-https://forums.overclockers.co.uk/threads/ebay-guessing-there-is-no-recourse.18789195/
- Affiliate Marketing Video Examples