Most of what OP has suggested actually puts you at an advantage of you're someone that never 'uses' minions, as now you're potentially in a party and the only one that can need on it is you, because everyone else has already acquired them.
And, I doubt they could even implement something to prevent sale of items you haven't 'acquired' without some huge rework of the game's items, so that doesn't seem likely to happen.
If everyone already has it, they can still use the greed button. A change like this won't suddenly plummet the supply of dungeon minions on the market board.
