People would bump their prices up to try to compensate for the % they would get taken out. I've seen it happen in other games with larger communities. Imagine what this small one would do.
The aim of tax is to remove excess zully from the game, not to help poor players become rich. 10% doesn't affect low level trades much, and if they need money, they can always put in some effort to farm. It's just a game, increase in price will not cause people to starve to death nor have any negative effect on their persons. One set of honor gear is currently 2b-2.5b (non hawker set), players bumping up prices will mean what? 2.2b - 2.725b? Is this few hundred mils significant? In my opinion not much.
Anyway, it's a free market based game economy. Players may bump up the price artificially in the short-run, but as money supply decrease, prices will have to come down to realistic levels eventually. The market will always fix itself.