You can find ports on the map. Two stand on Everon. Drive up to one and open the Vehicle Menu. On PC that is Shift+U. On a controller, hold the left trigger and press D-pad up. The menu offers Import and Export.
The port screen opens a cart. When you buy an item there, it goes onto the storage list of the vehicle you arrived in, not loose into a seat. You pay only for what fits, so a vehicle with room for fifty more items pays for fifty.
By default the list holds everything a non-vehicle shop sells, and nothing that belongs to the occupying faction. At the fifth level of Trade, or at a port near ground the resistance controls, the list widens to almost anything the economy prices.
The same screen also sells items out of that vehicle storage. A port pays only a fraction of the import price for an item. That fraction always sits under the local shop price, so buying shop stock and selling it back to a port never pays.
The extended catalogue works the same way on export as on import. The economy must price an item before you can export it at all.
A port price does not move with a town stock or stability. The Trade skill discount does not apply here either. Import charges the flat catalogue price. Export pays a set fraction of that price.
The port screen also carries a Resources tab, on both the Import and the Export side. This trades the four bulk construction materials: timber, cement, steel and hardware. See Resources for what they are and what they are for.
A resource price at a port moves over the campaign, unlike a fixed shop price. It changes at most once every six hours of game time and stays inside a band around its base price. The details panel shows whether the price sits below, near, or above that base.
Pressure from the occupying faction pushes prices up. Resistance control of a port pushes them down. Selling a resource back to a port pays half the going price.
A resource trade runs through the cargo store of the truck you sit in, not through the vehicle storage the item tabs use. The tab shows a resource disabled with a reason when you have no truck under you, or no room left in it.