Anyone who's played FFXI recognizes the importance of having various gil-sinks to help stabilize an MMO economy. Because monsters are constantly dropping loot that people sell, and because quests reward gil, money is generated out of thin air. If you don't have a way to dispose of it, the population slowly starts to have more and more money and the price of everything begins to rise higher and higher.
The way WoW primarily gets around it, even though their economy is historically far less important than in SE MMO's, is soulbound items that people drop a ton of money on and then trash or sell back to an npc for a fraction of the cost. It's basically a hidden fee. The way Square Enix does it is direct fees.