If you open up a cash marketplace there’s no way to 100% control it so that some profits go to CPG. In fact, adding a tax like that could very well make the official market less competitive than third party markets that wouldn’t have that kind of overhead for the seller.
Any marketplace or trading system would simply add a lot of complexity and costly stuff for CP to manage while also reducing their bottom line since people would be able to more efficiently fill out their collections (or, if you also remove the spirit system, it would also lead to ridiculous cost increases for cards). Net result is that CP would have to adjust current orb prices upwards and we, the consumer, gain nothing (unless you really like dealing with random internet people to get stuff you want, which I think most people don’t). And that’s assuming that the change doesn’t have unintended negative consequences like PR problems or lost time that could be spent doing something that actually keeps people playing the game, like making expansions.
The current disenchantment system already puts a hard cap on any card or set of cards you want in the game. No matter how valuable a card is the hard cap on 3x copies of a legendary is less than $20 (even with the new uncraftable expansion). For a card game that’s pretty reasonable.