How do I prevent a Sold item from being processed for purchase twice -- by accident?

[artbytonyt01]

I am new to Square. I am selling Original Art and prints. For an upcoming show, I have entered in inventory, each specific piece that will be in the show. If a piece sells I do not want anyone to be able to come along after and process a purchase of the same item thinking it is still available for purchase. How do I prevent a Sold item from being processed for purchase twice -- by accident? Is there an inventory 'marker/limit' that can be internally assigned within the system?

[Candlestore]

@artbytonyt01 ;

Is this for Online Sales or at the art event in person sales?

For online sales : https://squareup.com/help/us/en/article/5228-basic-inventory-management 

For POS sales I have only been getting a Warning that the items Stock level is 0 do you want to sell another one or do you want to add this item to the cart?  The cashier can say sorry this was sold or process the sale causing problems if it is unique like artwork.