The ports do not produce raw crafting resources, they only allow production of a given resource by players. In your case, another player sold hemp produced from his building or captured in pvp/pve (the latter is more likely)
yes, capturing a trader might provide sudden excellent trading options if you are lucky. Remember that almost all trading resources are unique to only one region now. Thus, if you were lucky to loot e.g. Jutland Amber somewhere close to the Gulf shore, you can sell it in a nearby port with a good profit.