The Trade Station:
The GUI looks like this:
Put an item in the larger slot (top-left) and press Buy Or Sell. If you don’t have enough coins to buy or if the output slots are occupied, the buttons will be grayed out.
Here is the GUI after an iron ingot has been sold.
Take the coins from the output slot and put them in the bottom-left slot to add them to the station’s internal coin amount.
If the station has enough money, you can even buy the item in the top-left slot by pressing Buy.
If you ever want to retrieve the coins from the internal coin buffer, press the buttons on the bottom. The coins should appear in the bottom-right slot for you to pick up.