When you know that you’re not able to get something, you want to know why. Even when you had absolutely no interest in the first place. Scarcity is extremely effective. (It’s the fundamental principle of hype culture.)

If the seller ran out of a product, that must mean a lot of people bought it, which must mean that it’s good. After all, if it wasn’t good, so many people wouldn’t have been fooled, right?

It’s the same idea of why people will join a line when they see a line, preferring to wait rather than visit a restaurant that’s quiet and empty. 

Clever entrepreneurs will use this effect to their advantage; for example, in the early days of The Hundreds, Bobby Hundreds sent his friends in to buy the few t-shirts he’d sold on consignment to the first store that took a chance on his label. The retailer came back and asked for more.

This is an opportunity to start a self-fufilling prophecy, to get your friends and contacts involved, and to get more people to start believing.