Well first of all slander and libel are torts, they come under defamation. So no it's not. This would be covered in contract law.
Have you offered him any personal information? Does he know your ebay account or anything like this? If he doesn't, I would just ignore him!
It sounds like you have rattled his cage by calling him out, and he is over reacting!
I would personally not reply to him, if he has no information of whom you are, then its probably best left as it is?