Agency trading desks -- the digital-media-buying platforms that have emerged over the past year -- may be the hottest new revenue opportunity for holding companies. But they face an often skeptical and confused reception from many marketers, as evidenced in a host of presentations Tuesday at the Association of National Advertisers Financial Management Conference in Boca Raton, Fla.

Presenters and questioners raised issues of transparency, kickbacks and agency self-dealing surrounding the digital-ad-buying phenomenon. Yet the biggest problem may be lack of information. In a fairly sophisticated crowd largely composed of corporate marketing procurement managers, fewer than 10 said they had extensive knowledge of agency trading desks.

Nor is there even agreement that 's what they should be called. Brian Lesser, CEO of WPP's Xaxis -- arguably the biggest of such ventures, with a claimed client list of 700 -- bristled when ANA group VP and panel moderator Bill Duggan used the term. He offered no alternative, though panel members used "programmatic trading" or "digital exchanges."

Whatever the trading desk are called, Forrester Research analyst Joanna O'Connell said they or something like them will ultimately become the predominant way that digital and some other media, such as addressable TV, are bought and sold.

One way of looking at the trading desks would be as the agency world's frenemy-style option to digital-ad networks owned by Google's DoubleClick, Yahoo Right Media and others that broker unsold digital display inventory in real time.

Agency trading desks use computerized "demand-side platforms" to do the same thing, but at less than the handsome markups of up to 70% ad networks get, Mr. Duggan said. Mr. Lesser said the agency systems also use advanced analytics to deliver the most appropriate audiences and return on investment. Mr. Lesser said Xaxis can tell, using anonymous data, whether someone exposed to a toothpaste brand's ad actually buys toothpaste offline -- though he didn't get into exactly how.

Other times Mr. Lesser said Xaxis may find what car someone drives is a better way to target ads than how much money he or she makes. But he was defensive about how the ANA has been kicking the tires of agency trading desks.

"I find it fascinating every time we talk about agency trading desks we talk first about the business model," said Mr. Lesser. "When you're looking at consumer products, how often does the consumer look at the product and say, 'Let's talk about the business model before we talk about the value that product is creating.'"

Yet transparency about the model has been on the mind of the ANA, which issued a white paper on it earlier this year at the behest of clients, Mr. Duggan said.

The transparency issue is "more perception than reality," said Brendan Moorcroft, CEO of Interpublic's Mediabrands Digital Exchange, who said he's happy to provide details to clients who ask.

Asked by Mr. Duggan about reports the ANA has heard of incentives or "kickbacks" offered to trading desks by digital media companies, both the Xaxis and Mediabrands executives said their policies prohibit such deals.

More broadly, Mr. Duggan said some marketers see trading desks as "double dipping," since clients pay agencies to manage media and then also pay agency trading desks a margin to buy it.

Mr. Lesser said Xaxis discloses how it works to all clients, all of whom use it for only a fraction of their buys, allowing them to easily benchmark the cost and performance against alternatives. Mr. Moorcroft said: "We get compensated for our services, for the intelligence or the analytics or the other elements of what we do as a company."

The two companies do diverge on another issue. Xaxis buys some inventory from top 20 premium digital publishers at a discounted rate upfront and resells it to clients at a markup after applying audience and behavioral analytics to find the best fit. Mediabrands doesn't do the upfront buys, only brokering the media in real-time.

Mr. Lesser said Xaxis turned to the upfront buys to give clients scale-buying leverage they otherwise wouldn't have in a pure real-time exchange model.

Mr. Moorcroft said one fear among digital publishers was that "this was just a play by agencies to rip the price floor out from media companies." But he said experience shows higher-performing media actually see improved pricing.

Agencies aren't the only new entrants to the digital ad trade. A host of independent providers offer similar demand-side platforms, and some clients in financial services and elsewhere have brought the process in house, said Ms. O'Connell in a later presentation. For example, Procter & Gamble Co., working with Yahoo, instituted such a system, called Hawkeye, two years ago, according to people familiar with the matter.

Some clients, she said, will never use a system where an agency won't disclose margins or details of how transactions work.

Speaking in an earlier session, Pivotal Research analyst Brian Wieser said the trading desks could help holding companies break out of the limit of 15% margins instituted by client procurement groups in recent years.

"It's fine for agencies to make more money," he said, adding that the question should be: "Are they helping you accomplish your goal better now?"