This is K2Store Pro feature
K2Store comes with a basic inventory system to manage your
product stock. Setting this will enable the feature.
You can enter the stock while creating the product. Once the stock reaches zero, the cart button will automatically be disabled and an Out of Stock label will be displayed.
You can allow your customers to back order the products that are out of stock. K2Store will show a notice to the customer when they order an out-of-stock product.
(Since 3.6.0): Enabling this will show the stock availability near the cart button