Many towns near the sea, or that have an airship station have trading docks.
You can purchase and sell cargo from these docks, each town has different prices for different cargo, allowing you to move your cargo about the world and make a profit for selling it at different towns.
To move an efficient amount of cargo you may need to invest in a ship or build you own to carry the cargo about the world. Some commands are disabled while carrying heavy cargo so you cant teleport, the cargo is just too heavy!!
You need to walk with the heavy load, or create a ship to carry it for you, the more you export the more you earn!
Check here for the current world trading tables.