To make a shop:

Put a sign next to, in front of, or above the chest. 

Skip the first line on the sign. (This will be filled in automatically if you do it right)

On the second line, put the quantity of what your selling. For example: a 2 on this line would cause one click to buy two, instead of just one.

On the third line, Put a capital B then a space, then the price you are selling it for. If you want people to be able to sell to your shop, use a S then the amount that you want to pay for the item. If you want the shop to buy and sell, separate them by a semicolon.

On the fourth line, write the name of the item, or the number that is the item id. If you are unsure about the name or the id, hold the item in your hand and do /iteminfo. This command gives you the exact item id to write on this line to make the shop work.

That's it! Make sure you have $10 before you start, because it charges you to make a shop, and if you did it right, your name should appear on the first line of the sign, and a message saying the shop was created and you have been charged $10 should appear in your chat.

Example: This is what your sign might look like before pressing enter. Ignore the line numbers

(Line 1)

(Line 2) 5

(Line 3) B 1500:800 S

(Line 4) Gold Ingot

This shop would allow customers to buy 5 gold ingots for 1500 dollars, or sell you 5 gold ingots for 800 dollars. Note that they can sell you items while you are offline, which can cause a large decrease in money if someone has a lot of an item to sell you. Feel free to comment here with any more questions you have about shops.

EDIT Also: About there being a place you can go to learn this, i'm pretty sure ssfdre38 set up a place at /warp tutorial that shows you how to make a shop.

Thanks Jupiter_Ginger for the instruction on how to go about setting up a shop. Very informative that others will too, find appreciative.