Marketing professors Duncan Simester of the Massachusetts Institute of Technology and Eric Anderson of Northwestern University did a study based on reviews posted on the website of a major private-label apparel company.
The duo found that about 5 percent of the product reviews were written by customers with no record of actually buying the item. Those reviews were "significantly more negative" than the others.
Story here. And yes, negative reviews adversely impact business. This is a great opportunity for an entrepreneur to develop a reputable ratings site.
BTW - I believe this is true of ratemyprofessor.com, a site that I never check. Not only are you likely to find selction bias, I know of a professor who sullies his own reputation on the site in order to discourage students from taking his classes. He gets paid the same, why put up with the hassle of more students?