This sounds like buyers remorse to me. As a seller you are pretty much screwed. Ebay will always side with the buyer sadly. Email the buyer and ask him to supply a letter from the camera shop which told him the lens was faulty, if he does just issue a full refund on return of the lens. You can always sell it again if it is ok.
Being a seller on ebay is becoming a real pain these days. If a buyer doesn't pay I can't leave negative feedback, but a buyer can leave negative for any reason they want.
Do not give him a partial refund and let him keep the item.
