Economy in FF cannot work with the way it is setup. RMT have characters in the game and they are using it to kill the market price to force people to buy to them after.
The fact the market price is only fixed by player without any minimum is not efficient at all. The system has to be based on an AI calculation that calculate a couple of criteria to fix a minimum price, and offer freedom to sale it with more or less 5%. AI calculation can be based on item level, complexity, number of people seeking for it and number of item in sell, etc....