The idea is not to make traders *required*. It is to make using traders more attractive. Fees on the order you propose would make trading completely unprofitable, even for experienced trader, and simply impossible for inexperienced traders.
Again, we are trying to control player behavior with the stick. Players hate this approach. We need to incentivize traders, not penalize lords.
I agree in theory, though with the current system lords have all the say. i.e. there is nothing really for traders to do when lords post few to no sell orders, and the price per bushel for sell orders are much higher than buy.
