Listen, I understand. And what the kid did and said was dumb. I get the principle. BUT...
All that was needed was a gentle and kind "No thanks I think my asking price is fair" and that's that. NO reason in the world to call me names etc. To be honest, I'd purchase the tank at his asking price after his denial, if he wasn't a flaming douche himself about things. It's a harmless, quick question, if the answer is no, that's fine. Just don't be a *****. It's not hard to treat others with some humanity. His tone since the very beginning was bothered and annoyed at the pure fact someone was even interested in his tank. Guys just an A$$ idc how you wanna phrase it.
That being said, i 101% agree with the points you made Helfrichs.