Universal Coins reborn

This mod allows you to trade any items using coins. The coins come in four values: the Coin (value 1), the Coin Stack (value 9), the Large Coin Stack (value 81), and the Coin Pile (value 729). Obviously, 9 of a lower-tier will get you 1 of the higher tier; also 1 of a higher tier will get you 9 of the lower tier.
The trading is done at a Universal Trade Station. The only way to get coins is by selling an item. Most vanilla items have a predefined price and prices can be changed in the config file.
Items can only be bought after the player has found at least one of those items (no buying blaze rods before finding one).

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.