The Market was added as part of the Gold Update.
The Market can be used to buy/sell items from other players, as well as selling items at their vendor set price. You buy the items from the players for Gold Coins. You can also put unwanted items in the Input area on the left of the market. A bar on the right side will appear. When it fills up, you get a coin. Certain items such as Spirit Key, raw gold, gold bar, and above cannot be put into Input area with exception of trading.