I didn't know there was such a rule, mods should pin it as a sticky in marketplace or classifieds.You are assuming the buyers are unsuspecting. I don't know who would be so, especially, when it comes to a high value item like GPU. As far what the MOD mentioned, this is what the rules say -
Technical and Supportive - It should add value to the discussion between the seller and the prospective buyer (all suggestions and negotiations on asking price and market price must be done via PM).
@puns , please clarify the guidelines.
That being said there's a limit to how much premium one can charge based on model, usage, how old etc but 40%+ just doesn't make sense.