The first marketplace is included with your city. The market place produces gold. When there is a gold coin symbol above your marketplace it means that there is gold ready for you. As you upgrade your marketplace the capacity for gold and the amount of gold produced per hour rises. In order to upgrade the marketplace your Headquarters must be at a certain level. There are 6 possible marketplaces to acquire during the game. The higher the level of your Headquarters, the more marketplaces you have access to.