I’ve been doing some research lately on U.S. legal requirements for using testimonials in your marketing.
Everyone should be familiar with these FTC guidelines.
There are some important requirements for testimonials that I see violated all the time.
Read the relevant info, especially DotCom Disclosures, found here:

FTC Information Page.
Also read this document specific to testimonials/endorsements:

FTC Testimonial Page.
And this [...]