People who play the market can't hurt anyone....

In fact, they are actually doing something that would be considered a service if they are successful. And if they are not successful, they fail miserably and lose their gil.

You only need to take 2 possible scenarios in mind. First, that a small number of items are for sale at lower than what the market would value it at. The individual who purchases and resells at a higher price, if the supplies are not quickly replenished at a lower price, then that individual has done a service. Without their action, those items would have sold for less and for less valuable purposes. It would have been first come first serve, and anyone who was willing to pay more would not have the opportunity to purchase.

When the individual is successful, they are merely ensuring that the items are going to those who value the item the most and are going towards the most valuable purposes. Their knowledge of the value of the items itself allows them to help others in this way, while those who list at erroneous prices lose only due to their ignorance.

Now the converse, if they are unsuccessful, noone is harmed and the individual making the purchases loses greatly.