High Sales = A large number of people can attest to the engineered good or service being of high enough quality that they will exchange hard earned money for the ability to use it.
Peer Review = Some folks who derive self-worth from citations ask you to add a citation to their work and you do it because you'll probably need to ask them for the same some time later.
Market success isn't the same thing as validity.
> Butt-candling[1] must work, just look all these happy customers!
But history is replete with ineffective or downright harmful treatments being popular long after the evidence showed them to be ineffective or harmful. Homeopathy is a prime example of this, seeing as those concoctions contain either no active ingredients or (in cases of low dilutions still labeled "homeopathic") contain ingredients picked based on notions of sympathetic magic ("like cures like").
[1] A hopefully fictional example.
