The guy is obviously a little naive but i dont think it was unreasonable for him to assume this part was also off the car you're breaking and selling parts from. We all have to start somewhere and he obviously isn't that knowledgeable and he assumed it would fit his car without thinking about it properly.

If you knew it wouldn't fit his car then why didn't you tell him to make sure he knew what he was doing buying it? yeah, it's not up to you to check but it's the decent thing to do, isn't it?

Refund and sell it to someone who can use it i say