Is it possible to just remove the back glass piece, so that the serial numbers will still match when I receive the lens back. How complicated is it to replace the back glass?
It's not hard. If you look at the 2 holes on the ring that secures the glass, this is for the tool used to unscrew it. Among the possible scam, he may had the same lens, damaged it, and bought one on
eBay, repaired his with your lens, and returned your lens with his damaged back glass piece. This sucks big time.
The cynic in me would say to purchase the same lens on
eBay, repair yours, and return the lens to the seller using eBay's favorable policies to buyers, but this is obviously not right. Don't flame me on this, I'm just half kidding! OK?
I hope you have some good pictures of your lens before you sent it to the buyer, so you have something for your defense. Hopefully some clear pictures of both business ends of your lens. Your other chance is with the insurance you bought from the shipping company. I would contact them, let them know what you think, provide them with your pictures, and let them deal with the buyer. They have lawyers, and they are used to deal with scammers, so hopefully they will handle it, not you. That being said, Paypal may reverse the payment until the issue is resolved though... That would suck again.
Note to myself : If I ever sell something on eBay, take zillions of pictures of the object itself, and several pictures of the packaging too. Buy insurance from the shipping company, and hope for the best.