Bukkit

Shops:

You can simply turn a chest into a shop by placing a sign above it against a block. There are different kind of shops based on what is written on the sign.

Infinite shops:

The most basic shop which can only be placed by someone with special permissions. All that is needed is the name of the item (as written in TradeCraft.txt) somewhere on the sign, surrounded by square brackets. You can place this on any of the four lines, so you could even write accompanying text on the other lines, like "Buy your/[Logs]/here!". These shops have an infinite supply of items (and buy an infinite amount, if not disabled).

Player shops:

These are built and used by players.

When placing the sign, you write the item name (as written in TradeCraft.txt), surrounded by square brackets on the first line. On the second line you write the rate you want to sell to customers and on the third line you write the rate you want to buy from customers. Both lines are optional (though you need at least one). The only requirement on these two lines is that it contains two separate numbers, you can write stuff around or between them in whatever way you want, preferably to make clear which line show what rate. You do not have to write your name on the last line, the plugin does that automatically (cutting off long names when needed - but the full name is recorded in the .data file).

The shop owner can add currency (when buying from customers) and items (when selling to customers) to the chest and then store them in the shop by right-clicking the sign. If items are already stored in the chest and gold is added, the items are actually placed in the chest (as the owner is notified in chat - see the image above). So be sure to right-click again to return the items to the chest. This is actually part of the following step.

The shop owner can retrieve items and/or currency from the shop by right-clicking on the sign. With the first click the currency will be placed in the chest. If the currency remains in the chest and the owner right-clicks again, the currency is deposited into the store again and the items are placed in the chest.

When a customer right-clicks on a sign of a chest (and there are no items in the chest) they will be informed about the rates and amounts of items and currency available in the shop.

A customer can buy something from a shop (if enabled) by placing the currency in the chest and then right-click the sign. If the amount of currency placed in the chest doesn't match the rate, or the customer tried to buy more than there is in the chest, the remaining currency will be safe in the chest. Be sure not to right-click twice quickly or you might sell the just received goods again, possibly against a less favorite rate, making you lose money.

In the same way a customer can sell items (if the shop buys from customers), by placing them in the chest followed by a right-click.