Trade Centers are built in the military sector of your city. At the Trade Center, you may exchange gold and resources. For example, you may use gold to trade for resources (food, oil, steel, rare metals), or vice versa, you may sell resources in exchange for gold. There are two types of trading that you may execute, either selling to the market merchant or other player characters.
The market merchants are not easily accessible; therefore, you will have to earn their favor or give them benefits in order for them to be willing to trade with you. The benefits of gaining the market merchant's favor is that there is no waiting period for you to receive your gold or resources.
You can also buy and sell your surplus resources to other players. However, resources purchased in this manner will require transportation time.