Your idea is also good - in theory. Some cards(IDK, solarisa) would need ridiculous amount of carats to be sold out. A limit would make some cards un-sellable.
What do you mean by bidding wars? Generally people don’t interact with people other than the ones they’re trading with. I think of it as a give an offer, not a bidding situation. We (unless I don’t know about something) don’t bid on cards with other cards usually, we usually just make a trade and are done with it.
I doubt people would hold cards for outlandish demands. Some cards are rare, in that case it’s not “Outlandish”. In other cases, it would be bad for people to sell thing for outrageous prices. Think of it like this. Why would you buy a card for 1,000,000 carats, an outlandish price, when someone else is selling it for 500,000 carats, a significantly reduced price, reasonably if pretty rare. Therefore, it is better for people to keep the price lower, that way they will actually get something.
But I agree that this might be good to moderate in some way, but not by rarity alone.