If you are working with the Professional version, go to Step 4 in the E-commerce section and open the Product Settings window. In the Availability section, you can choose from three options: Available, Limited Availability, Out of Stock. An icon corresponding to the availability indicated will be displayed in the Product description (which is shown in the product card that you created using the Product Catalog object or in the shopping cart). The appearence of the icons can be defined with the Product Availability option in Shopping Cart | Options.