Build Trade Module

[MichaelAGolden]

Trade Module is needed and is the most important aspect of the game. It hinges on a trade goods list, and functioning markets and cities.

The trade module needs to handle user buy/sell actions, adding/removing inventory and gold from user during purchases, and removing gold from city at same time.

Trade cycle should update economy at end of each day in theory, but for now we just need something working that the player can interact with.

[MichaelAGolden]

Initial trade module built, purchasing, inventory and gold are correctly updating now.