How about a Trade Center building where you can get rid of excess stock for other items at different ratios, based on value. Eg trade lots of low value to get a few high value. Goods aren't always available all the time (game driven), so you are still encouraged to build/farm your own. No coinage needed.
Also still need to be able to adjust stock levels in barns and warehouses at a local building level. Not just globally. This will then let you 'push' resources to where they are needed in your town.
Well done so far people, great game!!