MMORPG auction house buyers still have to deal with the market. Sure you can buy up everything and resell at a higher price, but all the player base has to do is not buy the item and the seller will be SOL.

A game designer solves this problem by charging a fee to list an item on the auction house, and limiting the time an item remains on the auction house. Thus if a player tries to sell an item for a price that is clearly to expensive and no one buys it, that player will loose money over and over again by putting it back up on the auction house.