Rosenblatt follows the golden rule of entrepreneurialism: Build a business on what you know. He understands this well — the one time he strayed from his comfort zone, things didn’t work out so well.

Serial entrepreneurs like Richard Rosenblatt develop new companies the way the rest of us shop for clothes: It’s just something we do now and again. One minute, it seems, Rosenblatt is ramping up an Internet company, the next he’s selling it to some giant corporation. And then he’s off pursuing a new one. The 37- year-old mogul has pulled this off so frequently, in fact, that you might think he’s dreaming up companies willy-nilly. He isn’t. As it happens, Rosenblatt is following the golden rule of entrepreneurialism: Build a business on what you know. And he understands this well — the one time he strayed from his comfort zone, things didn’t work out so well.

What Rosenblatt knows is advertising sales, a game he learned from the ground up. A lean, dark-haired Southern Californian with a ready smile and a gift for gab, Rosenblatt launched his first company as an undergraduate at UCLA. He’d been peddling ad pages in Los Angeles for a free weekly, the Village View, when it occurred to him that some of the advertisers might want to buy ads in other free papers around the country. So with his college sweetheart, Lisa (now his wife of 14 years), he formed R&R; Advertising, which in time grew to a 20-person ad agency. With his advertising experience and staff, he merged R&R; into an existing public company that later became iMall, the first online shopping mall. That was in 1994, the early days of the Internet. Rosenblatt approached selling Web sites as only slightly different from hawking print ads. Much of the young company’s revenue was, in time, generated in the seminar business, but when that operation flagged, Rosenblatt shut it down, for the first time earning scorn from friends and associates. “ ‘You don’t get rid of revenue,’ they told me, ‘and no one will pay for Web sites,’ ” recalls Rosenblatt.

Rosenblatt, it turns out, had something a bigger company wanted: his iMall owned valuable Web technology. Excite@Home, a powerhouse Web-and-cable concern, bought iMall for $565 million in late 1999, giving Rosenblatt his first big payout. His next venture, Greatdomains.com, created a secondary market for buyers and sellers of Internet addresses. “Everyone said it’s just a name,” says Rosenblatt. He ditched the unprofitable Greatdomains.com in late 2000 to a company called Network Solutions, now part of VeriSign, for $100 million

As with so many bubble-era investors, Rosenblatt’s luck ran out when the market peaked. He pumped money into the famously failing healthcare Web site Drkoop.com and became its interim CEO. “I started believing the hype,” he says. When Drkoop.com didn’t revive, Rosenblatt bought a home-infusion business in Detroit. He was there to meet his new employees on September 11. Shortly after that, Rosenblatt’s business entered bankruptcy, something he insists wouldn’t have happened if only he’d stuck with the advertising business. “Don’t get into things you don’t know,” he says, adding that as bad as the ad market was, he would have been able to operate his way out of it.

Rosenblatt got his mojo back and solidified his role as a turnaround maven in 2004 by becoming CEO of a down-and-out Internet marketing company called eUniverse. Lurking within eUniverse was a small project called MySpace.com. By 2005, eUniverse had become Intermix and was booming on the strength of MySpace as well as other sites that rely on content contributed by their users. Rosenblatt then sold Intermix to Rupert Murdoch’s News Corp. for more than $600 million. He’s fond of reminding people that when he joined eUniverse, several so-called social-networking sites were already flourishing. “Everybody said the game’s over,” says Rosenblatt. “Friendster, Orkut (owned by Google), and Tribe owned the space.”

Despite the temptation, it’s unlikely anyone will scoff at Rosenblatt’s latest venture, a company called Demand Media, an agglomeration of a little-known seller of domain names and a giant portfolio of underutilized Web addresses. “It’s a whole new kind of media company,” says Rosenblatt of Demand, which he says is short on employees and long on recurring revenues. Rosenblatt’s past successes give him the confidence to try something others think is nutty, but he believes the lesson translates. “You’ve got to believe in yourself and that you know your business better than your critics do,” he says. Can we doubt him?