Market is a building that allows you to either sell gold for other resources or selling resources for gold.
There are 3 levels of Market, Market I, Market II, and Market III. The last one requires 10 friends to be hired in order to open it. There is no big improvement from Market II upgrade to Market III. Player can only trade 20 times each day.
There is also Allies Market that can be bought too from the shop, they give you resources one time each day, more resources can be got by adding up to 10 allies/friends and you will recieve about 150 of the resource you choosed from each ally that answer the call.