Step 1: Making a Vault, this is a special chest that stores any coin you place in it, makes it pseudo-virtual where it doesn't clog your inventory and you can see your vault balance with the /money command. This is performed by placing a chest down, attaching a sign onto it which reads [Vault]. You will get a confirmation message if you do this successfully.

Step 2: Place any amount of coins into it, these coins will be used when people try to interact with your store while you're offline. When somebody sells to you, these coins will be reduced, and when somebody buys from you, they will be added into the vault.

The player name gets automatically added into the sign, it doesn't need to be written down. Also, for selling lored/custom items: Do /iteminfo with the item in your hand to find out its proper ID that's compatible with the shops.