FOMO, or the fear of missing out, is a psychological trigger and a popular marketing tactic usually reserved for customers when they are browsing online. It is the fear that something is going to sell out due to limited availability. We all have this fear, and it stays with us in our day-to-day interactions. Sometimes you might not even realize it's happening.
For example, grocery stores will often use this tactic to sell products that they need to get off their shelves. They accomplish this by putting up a "limited stock" of one item, while a similar product may be stacked up right next to the scarce product. Your instincts tell you to go with the item that looks like it's selling faster because you don't want to miss out.
You can apply this same tactic to your products after a customer makes a purchase. Usually, the purchasing process involves a customer signing up with their email address. You could tempt that customer to become a repeat customer by offering exclusive 'limited time' deals on your products.