Some products are not currently sold by merchants. Planks, Flax, Paper, Books
Inversely, not all products are bought by merchants. Cows, Willow, Flax, Paper, Books
This makes sense for testing purposes, but ultimately for sense of realism you would expect other tribes to also want planks and cows and whatnot.
If they’re sending a guy on a journey around the world to buy your potpourri (soap herbs) why dont they want some books or maybe a farm animal?