Persuasion techniques: Scarcity and social proof
Yesterday I was at the grocery store shopping for necessities, under strict command to buy "just what's on the list". As I walked by the peanut butter shelf, I saw one last jar of Smucker's creamy peanut butter. Not on the list, but I bought it anyway. What's going on here? Fear of running out? No, we had a full jar at home. Righteous independence? Maybe. Scarcity? Definitely.
Human psychology increases the value of an item when there are few left; that's true whether on the shelf or on-line.
Many sites use inventory to trigger a scarcity response. It's common to see "Only 3 left in stock" proudly declared with an item's description. It also appears on the search results.
Here's another psychological technique used to encourage sales – social proof. If others are buying something, then we are persuaded that we should, too. We see this clearly when hundreds of others have written customer reviews for a product, and we think to ourselves, "Everyone likes it, it must be good!"
Used well, these encourage browsers to become buyers. And it helps them finish their shopping in time to enjoy the holidays.