Social Proof is a psychological phenomenon that causes us to like what we perceive to be popular.  Social proof helps a customer be more at ease shopping with you.  Your customers are influenced by people with similar tastes, and they’ll automatically find people with kindred needs in your sales history.

You can help your customer find social proof on your business by using phrases such as “best selling” and “back by popular demand”, and “previously sold out” on items that have a history of doing just that.  Social proof can come from brand recognition or customer reviews and comments about your company or products in social media.