This is one of the domestic orders players may issue from a nation.

The merchant is used to buy and sell resources at a nation level. If you find yourself in dire need of one resource, you may give a merchant order and buy food, wood, metal, or stone at the cost of gold. If you find yourself with an abundant amount of a resource, you may sell it directly to the game by giving a merchant order receiving gold in exchange for those resources.

The merchant only deals in units of 100 resources, therefore you may only buy or sell 100 resources per transaction.