In Age of Empires III, a Market is an econimic building that can be built infinte times. A Market is where you buy resources you need but don't have, and you sell surpluses. You can also upgrade in the first three Ages. When you buy logs or meat, the prices go up. When you sell. they go down. Markets have no defense, so during a battle, post ranged infantry, light calvary, or falconets around them just incase you need to get coin, logs, or meat quickly.