Commerce makes it easy to provide your citizens with the goods they require but you don't produce yourself.
All commerce involves transport of goods. When trading on Merchants' Den or importing goods at the Trading Office, you need to have at least one available trade portal, otherwise transport of goods is not possible.
The Trading Office
The Trading Office (part of the Commerce Adviser) is the place to go to when you're looking to import or export resources for gold.
Exporting resources is a great way to add to your income.
When at the Trading Office, it's always the buyer's responsibility to ensure transport of goods bought.