Most investors can’t tell the difference between “sponsored investment research” and independent analysis, and that’s exactly what the “sponsors” — typically small companies paying for a marketing campaign that will inflate their stock activity and value — are counting on.

The difference gets even tougher to figure out when the sponsor hires someone who is known for giving independent commentary colored only by his own feelings and research.

Think of it as a big, honking commercial, with a celebrity endorser.

Last week, that bought-and-paid-for stock endorsement was a 20-page mailer about Petrosonic Energy PSON…supported by an email campaign, featuring Tobin Smith, a money manager who has been a fixture on the television news shows for 15 years and who is a regular on the Fox networks, describing himself on Twitter as a “guest anchor.” According to Fox, he is “a contributing market analyst for Fox News Channel and a regular panelist on ‘Bulls & Bears.’ ”…

The people who contacted me considered buying the stock entirely based on Smith’s say-so, and the credibility he exudes in his Fox appearances. They didn’t appear to read the disclaimers of the campaign; had they bothered, they would have quickly found it was paid advertising for which Smith’s company pocketed $50,000.

This is a unique case because of how recognizable Smith is, and it brings into focus credibility questions that I’ve found most investors fail to answer even as they are deciding who they can trust. Fox’s official policy is that “no Contributor to FBN, nor his/her firm, and/or family members are allowed to accept financial consideration of any kind whatsoever to issue research, advertisements, or to otherwise promote individual stocks or securities,” a spokesman says. However, the network, which was unaware of Smith’s efforts on Petrosonic’s behalf until I contacted them, would not comment on the matter.

On Tuesday afternoon, though, Fox terminated Smith’s contract on the basis of its contributor policy, according to a spokesman. Smith, by text message, confirmed the news.

Asked earlier whether he had violated Fox’s rules, Smith said, via text message, “that policy was added late last year … my contract was not subject to that clause …”

I guess questions of ethics and legitimacy never entered discussions when the decision was made to take the money and run.