How do I prevent sales from quantity table when product is less than 2

How do I prevent sales from quantity table when product is less than 2. More details: I have a sales page and a quantity table, when a user makes a sell from product table that has the quantitys of product, brand and category of products it minus the quantity by customer from the product quantity. My challenge here is this, if the product quantity is remaining 1 and the one is now sold, the application will still be selling from the quantity in product table and you will starting seeing minus 1(-1, -2). So I want the application to display message that the product is remaining 2 and once the product is 1 it will prevent the user from selling further. Please any idea on How to achieve this?. thanks