Commerce can be done internally, using the market, or externally, by trading resources with other Lords. In Economy => Send is the interface to do this. Simply chose the amounts of each resource and the destination Land, and then use the “Send Resources” button to confirm your options.
Resources sent in this manner will take a number of days to reach the destination Land that varies according to the distance between the two capitals.
Note that players are free to negotiate their commerce, and are also the only responsible for maintaining/fulfilling their agreements. The only thing players aren’t allowed to do is voluntarily sack their Lands in favor of another Lord, since that is considered to be against the basic ethics and sportsmanship that must prevail in the game. Infractions will cause the anger of the Gods (here represented by the Game Masters)!