I've been using coins for trading for a while now and I love it. One of my goals for this game is to make coins the default trade currency with our stands! Muahahah... er, anyway I had some ideas to improve stands
Shift+scrollwheel item adding: You can already shift+click from your inventory to a trade in your stand, but if you have multiple trades with the same item only the first one will be filled by shift+clicking. My proposal is to allow shift+scroll on the trade and it will move the item in/out of your inventory. In the case of putting a stack of coins on a barter stand shift+scroll will pull the proper amount of coins from your inventory (no more need to split stacks to fill your stand.)
Multi-coin type trades: For vending stands add two more "coin type" payment options: "bronze or cast coins" and "bronze, cast, copper, or tin coins". Similar kinds of trades are already available for metal bars on the barter stands, but my suggestion has one addition: wrought, steel, gold, and silver should NOT be accepted with these options. Cuz you're gonna be pissed if a gold coin gets pulled from your inventory in this case!