To create a shop to sell things to players you first want to place down a chest. While looking at the chest type /qs create with the item you are planning on selling in your hand. It will then prompt you for how much you want to sell it for. Type in the amount you are selling one of that item for! It will then create the shop!
If instead you want to buy that item from a player you can type /qs buy while looking at the shop that you previously made. This will convert your shop into buying mode which will allow players to sell the item specified to you! To switch it back to selling mode you may type /qs sell
When you are finished with creating a shop you must stock your items in the chest!
/qs create - creates a shop
/qs buy - changes the shop type to buying
/qs sell - changes the shop type to selling