Besides selling items to your guests, you can make a sale from Point of Sale, which is not connected to any reservation in the system. 

To sell an item to a non-guest, open the POS, select items and simply click on Pay Now to register the sale. You don't select any guest or enter the person's details in the Search guest field, you just make a sale without assigning it to anyone.

You will find this sale in Accounts, under POS Sales category and in the sales reports.