They make time the scarce resource. They often have a counter on the screen, too. Sometimes the counter runs down with every sale. We only have a limited number of these fabulous quilted party skirts and when they're all gone, we will never sell them again. They make the product scarce.
Scarcity is a time honored tactic.
Limited Time Only. The Weekend Special. Sale Ends at Midnight. Here's a great one from Olan Mills, the photographers. They will take 10 different pictures of your child.