I’ve had a trade port open for over 30 in game years now, and only 4 merchants have sold anything other than Daler and Penning. The custom order list shows all of these many merchants can only sell these two things.
That’s a bit unlucky. The ratio of import to export merchants arriving at the big trading post is 50:50 and those who bring money just want to buy stuff. This one would pay the best price for furs and hides.
If you don’t like to deal with coins or this import-export approach, you can build small trading docks. There, merchants come to barter trade with your people and always have a bunch of useful resources on board.
I’m currently working at a trading post with a dedicated export dock for better control over random merchants without removing the money feature.