In the shop you have 2 shops. The playershops and the AdminShop.

AdminShop

The AdminShop is the "main" shop. This is from the server itself. Here you can buy and sell lots of stuff. This shop is quite pricey. Through the signs in the shop you can buy and sell things.

PlayerShop

The PlayerShops are shops that you can rent. Everyone can rent a shop. If you have rented a shop, you can all sell / buy stuff here. Usually the PlayerShops are cheaper than the AdminShop because the players decide the prices themselves.

You can place a sign on a chest to buy/sell things. In the chest you can place the things you want to sell. Type on the sign:

First line: *your Minecraft username*

Second line: *quantity*

Next line: *B price or S price*
B is buy and S is Sell
If you want to use both use: *B price:S price*

Last line: *item name*
Example: