Get a chest. Get whatever you want to sell. Get a sign.

Place the chest. Place the sign above it, or next to it.

The sign should look like this:

1st line: leave empty
2nd line: 1
3rd line: B:(Price you want to sell 1 of the item for)
4th line: The name of the item.

Then place the items in the chest. You should get a confirmation message when you make the shop.

You can change the number on the second line, but then that becomes how many you want to sell for that price. So to sell 32 cobblestone for 64 dollars your sign would say:

32
B:64
Cobblestone