“As a widow, you find you were so dependent on your husband, and when you make a mistake because of predatory businesses, it’s embarrassing,” Ms. Lankford said. “We’re an easy target.”

“Easy target” might describe the audience for several enterprises stamped with the Trump brand that have been accused of preying upon desperation, inexperience or vanity. Some are well known. Trump University has most recently gained notice because of Mr. Trump’s attacks on the Mexican heritage of the judge overseeing a fraud lawsuit brought by former students. There was also a multilevel vitamin-selling enterprise, the Trump Network, that Mr. Trump had said would give hope to people looking to “opt out of the recession.”

But intersecting with these was another, largely unexamined, business venture, Cambridge Who’s Who, which generated hundreds of complaints that it deceptively peddled the promise of recognition in a registry, as well as branding and networking services of questionable value. Dozens of people who paid Trump-endorsed businesses were also sold products by Cambridge, which benefited from its partnership with Donald Trump Jr. through “leveraging relationships built by the Trump empire,” according to Cambridge.

Cambridge was not a Trump company; it was operated by Randy Narod, a Long Island, N.Y., nightclub and bagel store owner barred from the securities industry for having had an impostor take his licensing exam. However, Cambridge gained the Trump imprimatur when the younger Mr. Trump came on board in 2010 and began promoting its services as a way for people to distinguish themselves in a tough economic climate.

He worked in plugs for Cambridge during interviews on the Fox Business Network and TheStreet.com, did a promotional video and appeared in photos with Mr. Narod. Among them was one with another Trump executive at Trump Tower in New York, where, according to a news release, the three men discussed “strategies to expand the personal branding and professional networking services offered by Cambridge Who’s Who.” Mr. Narod’s company said on its website that it had embarked on a “global expansion with the Trump Organization.”