But allowing you to play treasures apparently takes up 0 space on the card, why would allowing you to play coin tokens take up any more than that?
If you're not just being hilarious, playing treasures does take up space, obv. obv., you just haven't seen that version yet.
Black Market dates way back. Early on Buying cards implicitly let you spend treasures, and they were spent not played (and there was no Buy phase). Some Prosperity cards said "when you spend this." That was confusing, e.g. when the card made $2 and you spent it on different things. Later I changed it all to the way you know it, but Black Market slipped out.