So the buyer now claims that the item is defective. While it may perfectly be the case I have reason from all my transactions with him to be skeptical.
If the item is returned and I see no defect, can I dispute it? ( An eBay rep just told me that not )
No, but you can report him for a fake claim once he returns it, will help future sellers. My listings are very clear, so I tend to block buyers who ask too many questions, because they are usually customers you can't satisfy no matter what. If the buyer creates a new account to circumvent the block, and you can prove it (for example new account was created shortly after block and shipping to same city), you can then cancel their order and/or remove any negative feedback they leave.
You got screwed and it happens. Here's one I had last week if it makes you feel better. Guy in Peru ordered a brand new, manufacturer sealed $400 speaker. He put some dirt on it and claimed it arrived like that. I realized I got scammed the second I saw it was a new account with feedback from cheap purchases from Chinese sellers. Nevertheless I accepted the return and told him I can't send him a label so he should send via the cheapest option and I'll refund him. The guy knew his stuff so he tells me that DHL charges $600 hoping that I'd tell him to keep the item. I then went on the local postal website and got an estimate for $170. He complained it would take too long. I called eBay and they agreed to move forward with the return if I attached the estimate and sent him the $170. I sent him via PayPal goods and services so I could cancel because I knew that he wouldn't ship it back. He still managed to complain to eBay and they refunded him at no cost to me and closed the case in my favor. I then fought the $170 charge with PayPal and won the case. I'm upset eBay gave in though.