Well, his opening remark on the forum ended in "wondering what kind of piece of shit I just purchased here.."

That appears to be before any contact with anyone at all.

That, alone, doesn't cross a line. But it is a clue that the interaction might quickly lead down a path where you choose to part ways.

I'd have been initially polite, helpful, try to fix the issue, etc. But there would be some line where I'd stop.