The Foreign Traders are the foreigners of which they speak (they are representative of Vikings, who were actively trading throughout the area for centuries). Furs are most valuable to them, with extra value to the small pelts that are more decorative than useful, because they would trade them with the Britons and Franks who had long since depleted their fur bearing animals, and receive high quality steel and other raw materials in exchange. Driik are considered the big traders because their lands represent the entrypoint from Scandinavia into Finland. I do like your mention of pirates, a) boat using NPCs at all and b) potentially hostile foreign traders, those guys are very well equipped and run pretty big groups.