NPC trader

We now have an NPC that basically mediates our currency market.

But would it not be a good idea to have an NPC that trades currency between the player and the game itself instead of between players ?

It could for example be set to take 300 exalts for 1 div and 1000 divs for 1 mirror. 1 directional. Or maybe it would have a diferent rate in the other direction like it would give 100 exalts for a div and give 200 divs for a mirror. That would put a cap on the exchange rates and it would adjust the supply in the market of these products. (it would basically put more mirrors in the market by the end of the league while taking out the lower denominations. This way the lower denominations would maintain some value for the casual/poor player while making the true end game more accessible for the active players)

I'm sure the devs have considered this. I wonder why it is not happening? What bad results am i overlooking ?