Seems as you found a bug in the old North. I made a patch to fix this. You can download it right here: MarketPatch.zip (2 KB). It has to go above the North mod to overwrite the market storage (turns red in mod list) and you have to rebuild the marketplace if you want to fix it in a running game.
I used this opportunity to change the market in a way it was suggested some time ago. Vendors will now more focus on food and not carry iron, ore or fuels there anymore. Also no herbs to not steal them from herbalist cabins.