I sell a lot of stuff on ebay and have sympathy for your problem. I was caught out by a dishonest lying toe rag of a buyer when I sold a spare door lock for a Range Rover.
The vast majority of transactions go smoothly (I have a 500+ rating and just 2 issues) but if a buyer wants to be an arse, even if you have done everything right, there is not a lot you can do about it.
Although I knew the part I sold was working perfectly, he claimed it was broken. He returned it under the seller protection policy, except he actualy returned a tin of beans, and they were not even Heinz
After a lot of emails, he got his refund and I lost the selling price, the postage and the lock. At the time I was really angry, with time I have accepted its just the cost of selling on ebay. I would be a lot worse off without ebay, so I accept every now and again I will get my pants pulled down.
What I dont understand is how he has managed to raise a case ? If its outside the 30 days from payment he should not be able to do it. Is it a case through resolution centre or is it a personal ebay message ?
If its just a message, email him back saying the part was 100% and then ignore him . If its through the resolution centre, contact them immediately to find out why he has been allowed to raise a problem so long after it was sold, dont ignore them, they will simply take the money back from you anyway.
Good luck, I hate tossers and what he is trying to do is basically theft in my book.
P.S. I have an Etec bought new in 2006, the decals are looking a little faded, can I send it back for a full refund ;-)