First method of buying or selling gold are through re-sellers. Re-sellers are people that buy gold for lower price and sell for higher price. To buy gold from a person like this all you have to do is click on any field in main menu except for "home", for example "inventory" and on the left side of the screen next to the bag with your gold click on "+" or "-"(doesn't matter) with the description "Buy gold for cash" or "Sell gold for cash".