reminded the dickhead that "near mint" to me didn't include bent knobs, scratches dents etc etc, and he emailed me back saying that maybe I had better think about what " near mint " meant on a piece of 20 year old audio gear and that he would refuse the amp if I left negative feedback for him and I would be stuck with the POS amp.
I think the problem with him, outside of being an ass, is in the middle of the comment above.
That being "I had better think about what 'near mint' meant on a piece of 20 year old audio gear."
Ratings of condition are applied to things current and ancient. I don't think the idiot realizes that "near mint" means "near mint" regardless of age. The ratings apply regardless of age.
Say you are a collector, where condition means the most. Could be early records, coins, or baseball cards. You have an extremely, extremely rare coin, card or album where there are only four or five known copies existing in the world.
Even in that situation, near mint means near mint. Even if the existing copies are in horrid shape, their rating is described as you would describe anything else, regardless of the value they command, regardless of their age.
Sounds like you have a seller that doesn't grasp the concept of ratings and describing things as what they are. Whether he understands this or not is his issue to resolve, as it is false advertising, regardless of his ignorance, but has unfortunately become your problem to deal with as well.
Had been advertised 'as is', or not mentioning a specific grade, but leaving an inquiry for more detailed description or pictures, I would disagree with you and suggest 'buyer beware', but in this case, the simple fact that he decided to rate it negates this in my mind.
I'd continue the dispute. Remember that if he's this ignorant to ratings, he's obviously an individual, and obviously someone new to the selling game. If he refuses, find some nice references to ratings, and if he ends up refusing any deal during a dispute, mention that you're sending him a lovely Hallmark card for the 4th of July from your lawyer. I doubt the dispute will continue upon mention.