When things are scarce we want them. This principle is a tactic used by several marketing professionals. To bolster the demand of their product, this principle is used.
Ever saw something you were not entirely sure to buy and the webpage or salesperson said that it is running out of stock? Did you then, buy it? The website used this principle to push you to buy what you were looking at. Several popular e-commerce websites use this tactic to pursue the consumer to buy a product.