Almost every game with trading function needs gold sink, otherwise item prices will exponentially increase by time.
And at some point (like 1 week after release) you can’t reach any item by grinding, because you need more time for grinding, but item prices will increase much more at the same time. It makes the game “either play the game at first week, or don’t play at all”.
And this makes trading guild useless after 1 week of league release. Player retention will be so low, because late comers won’t play merchant league at all. And people want to play game for longer time (like months) period won’t have incentive to do so.
Also party players are big source of surplus gold, since they can gift each other any item, and they are much faster and powerful, without any downsides like less loot.