Taxing feels bad, and it could just make things worse (e.g less people list items, easier to manipulate markets).
Imo, just add price ceilings and floors, so in the worst case we know the max price that newcomers will have to pay to get deck x or card y.
I like trading, its a big part of why I like the game, but the market is very punishing to players who don't follow it and I don't think newcomers or others should be punished for not wanting to invest time into the market.