It's done. I let him off, and told him he needs to pay attention to detail because the next guy might not let it go, and that trying to BS me by saying that it was for gaskets was low.
Come back and let us know if you get any type of "Thanks" from the seller. My bet is that you won't.
A seller that made an honest mistake will thank you for what you did. A dishonest one will just smile and move on.
You're a good guy for what you did.