There are already merchants in the game. I think these should be able to be broken and made to trade goods for their owners.
As suggested above there would be a craftable station. Give it HP amounting to about 200-300k.
Once placed would only be able to run with merchant thrall. Two slots in main GUI at the top and invantory slots below decided into giving and receiving. To guests these invantory slots will always show empty. To the owner all invantory is visable. First slot would be item you are looking for be it gold coin, bar, hide, bark, ect…
The second slot would be the item you are offering in exchange.
The number of items in each slot is the ratio of the exchange.
Example: slot one, 3 steel bars. Slot two, 1steel reinforcement.
For every 3 steel bars given, you gain 1steel reinforcement.