Who Pays for the Site, and Who Pays the Reviewer?
For someone seeking reviews of automobiles, the choice of www.autotrader.com isn't as unwise as going to single-brand sites, but it's not at all free from the chance that the reviews are being influenced by the behind-the-scenes business relationships between the website operator and the advertisers. Click on the "About" button at the top of the home page, and scroll down to this information:
Advertising Opportunities on AutoTrader.com
Local, regional and national advertising programs, specifications
Business Opportunities with AutoTrader.comAffiliate site opportunities with AutoTrader.com
Marketing opportunities with your company for AutoTrader.com
Strategic business partnerships
Of particular interest are "marketing opportunities with your company" and "strategic business partnerships." Those are terms of art in the business world for the establishment of agreements between companies to promote each other's products over those of their competitors. The consumer rarely knows about it, but simply sees the good news about the partners' marvelous products magically "bubbling up" in the website content. Bad news about partners' products tends to disappear. You probably know that this hidden business relationship also takes the form of those banner ads that fill portions of the screen at strategic moments. It's not illegal, but it's a likely source of distortion in your evidence. You could still use their reviews, but you should cross-check them against reviews by sites with no commercial ties to the automakers. That will enable you to say you really have good reason to be confident in their truthfulness. The same tests would also allow you to check on the reliability of other commercial sites which might offer reviews of cars, like Consumer Review's CarReview.com (a commercial site whose name deliberately mimics that of Consumer Reports, a non-commercial site--link requires subscription) and Road & Track.