People are more likely to buy a product or service if they know other people have already bought it.
How to use it:
Gather testimonials, case studies, shout-outs, and reviews.
Then include those in your emails, website, and social accounts.
Example:

Many underestimate the generosity of (most) publishers. I probably got $10,000 worth of free books during my 4 years in Durham by request exam or review copies. Sometimes, I just emailed a publisher and said, "I need this book, but I can't afford it. Can you help me out?"
— Stephen D. Campbell, Dr. theol. (@the_OT_Campbell) December 13, 2020
Old is Gold....
— Professor (@DillikiBiili) January 23, 2020
this Bharti Airtel chart is a true copy of the Wyckoff Pattern propounded in 1931....... pic.twitter.com/tQ1PNebq7d