Your system should communicate backstage events that may impact users, with notifications, indicators, and/or modal dialogues.

For example, to encourage future purchases on a site, it is good to properly communicate how far away that user is from qualifying for a deal.

Another example: when stock is low, this should be indicated. This indication might prompt users to act quickly according to the scarcity and social proof biases.

Additionally, though, if no items are left in stock, your system should communicate this too in order to save the user effort, because otherwise your site will lose credibility and compel users NOT to perform further actions on your site.