Put yourself in the buyer's shoes. They bought what they thought was a genuine item, only to receive a fake item. Would you get angry if you were in the same circumstance? I think it's reasonable to get angry in that sort of situation. It's up to you to know what you're selling. Whether you sell fakes intentionally or because of incompetence, it's not fair on buyers so you have to expect to cop some flak over it.
As rocket2retro said, I don't think ebay require fake items to be returned so just refund and move on. I could be wrong but I think I've read that it's illegal to even send counterfeit items through the post.
