I'm assuming you mean a chest shop?
For that, you'd place a chest and put a sign on it, and then in the following order put the information on the sign without the brackets:
[in game name]
[quantity]
[B/S (price)]
[item name]
B = buy = the customer buys
S = sell = the customer sells
You can make a buy and sell chest by doing [B 50 : S 50]
Some items have super long shop names, so if you hold the item in your hand and do /iinfo there will be a shortened name that shows up in chat. Type that for [item name] and it should work properly.