Gil-selling isn't linked to underselling. RMT doesn't add gil to FF, and adding gil would tend to have a price-raising effect anyway. It's gil-sources that create gil into the game. Item drops don't do it at all until the item is sold to a vendor. Even crafting and selling doesn't create gil into the game. Only selling it to a vendor does.

It is probably all the quest, dungeon, and fate gil-rewards that are the biggest sources of new gil. Selling a new item to another player creates no new gil. Each market transaction is actually a gil-destroyer. And crafters are just gil-accumulators. Forget the real money in RMT. All that happens on the server is that a crafter's gil is transferred a couple of times to end up in another account.

Prices are going down because free-drops have outstripped current demand. And the price of a free-drop that exists in massive quantities should be near zero anyway. Eventually, it will level out again near some equilibrium, until a new big patch.