In order to create a player shop you’ll have to first place a container below the block you want the shop to be. This can either be directly below or with a one block spacing.
After that you need to create a Villager to be your Trader. Type /vshop create minecraft:villager [Name your Trader]
To add Items to your Trader hold that item in your Hand and type /vshop add [Price to Sell to other Players] [Price to buy from other Players]
The chest should be protected from punching, explosions and is only accessible for the owner. Double chest wont work - The plugin does not care for the other half, neither stock nor protection wise.