Take the popular argument for Straws for example, if you threw straws into the Paramarket for 3k zeny, botters with obsurd amounts of straws would just readjust the prices and still make profit, the slight decrease of zeny per straw would be made up for by just running an extra bot. So what do you do? Sell straws in paramarket for 2k? Oh, then its just going to be readjusted so the botted straws are where people buy from anyways. Eventually you reach the point where you might as well hand them out for free because it will be the only way to deter bots.