This is the point I think you may seem to be overlooking, forget about the RMT spammers and bots for a minute, imagine if most people simply did this. Gil influx would be through the roof, sure you might get more on the market board but a lot of people would say screw it, easy quick cash. That much gill being introduced into the economy on top of the bots, would be an absolute nightmare, sure the prices would go up on everything, but that's because the buying power of gil would be low as its become worthless and there is so much of it in circulation, hence the need for gil sinks.
In a nut shell, you can't solve economy problems by simply printing money. Doesn't work in real life and doesn't work in game.