To Expand on what Artbot said:

For the third line, if you are selling items to other players, you need a B before the number.

My shop would look like

>Jupiter_Ginger
>64
>B 32
>Cobblestone

This would sell other players 64 blocks of cobblestone for 32 dollars per transaction.
You place the sign on/next to/above/below one of your chests, and then put the item you are selling in that chest.

To get the item ID, hold an item in your hand and do /iteminfo. Then place that number on the 4th line instead of the name of the item you're selling.

To make a shop that other players can sell their stuff too automatically, replace the B with an S. My shop would look like:

>Jupiter_Ginger
>64
>S 32
>Cobblestone

This shop would allow players to sell me 64 blocks of their cobblestone for 32 dollars of mine per transaction. Be careful with this, as it will take money out of your balance without informing you when it makes a sale. It only works if there's enough room in the chest that it connects to.

You can also combine both in a single chest by typing things like:
B 32 : S 16
This would allow another player to buy the item for 32 and sell it to you at 16. Just remember that buy and sell are from THE OTHER PLAYERS point of view. So buy is them buying, not you. The same goes for sell.