By day, David Marlin toils in the marketing department at American Express Co.

By night, he designs "Shoppertunity," a Web site that lists companies on the Internet where consumers can make secure credit card purchases.

It's truly a double life, as Mr. Marlin lugs a mobile telephone and laptop computer everywhere and stays awake until 3 a.m. to troubleshoot the Web site.

"I'm a heavy technology person and I use the Web all the time," said Mr. Marlin, 29. "I decided about a year ago to use it to buy stuff, but I didn't realize it's about as inefficient a way to shop as there is."

Last November, after trying in vain to find a place to buy a scarf over the Internet, Mr. Marlin decided to build a directory for what is, in essence, the world's most chaotic mall. He said he and three friends spent "hundreds of hours" surfing for shopping sites that used Secure Sockets Layer-a technical protocol that assures at least some level of safety in credit card payments.

"We're very leery of a lot of the sites that claim to offer secure transactions," Mr. Marlin said.

The referral site, www.shoppertunity.com, went live in February. This month it reached a "critical mass" of 60 companies listed, Mr. Marlin said, and he began a modest advertising program.

The retailers are divided into four categories: books and music; clothing and accessories; electronics; and flowers and gifts.

"We're adding new sites all the time and taking ones off all the time," Mr. Marlin said. "Even in the course of a month, sites go out of business, they change their address ever so slightly."

To give an example of how hard it can be to find a site on the Internet, Mr. Marlin recalled a guest lecture he gave at his alma mater, Columbia Business School, about building a Web business. When students later complained they couldn't find his site, he registered a second domain name- "www.shopportunity.com"-for people who viewed that as a more natural spelling.

"There isn't a lot of reliability on the Web, and we want to bring some reliability," Mr. Marlin said.

Like others in or near the credit card field, Mr. Marlin is now looking to Secure Electronic Transactions-the more card-specific encryption protocol developed by MasterCard and Visa-to draw shoppers and vendors to the Web.

Among the conductors of numerous SET pilots is Mr. Marlin's daytime employer, American Express.

With SET not widely available-it will take time to deploy the necessary digital certificates among banks, merchants, and consumers-SSL is considered the safest way to ensure card security on the Web. But as Mr. Marlin discovered, the number of sites with SSL are relatively few, and transactions with them can be frustratingly slow.

"With the Web as it stands now, shopping is not going to go anywhere," Mr. Marlin said. "People have to find the Web to be a quicker tool, otherwise it's faster to jump in your car and drive to Bloomingdale's."

He believes his venture has "a lot of potential" to be profitable. He built the site and did all the coding himself, so it will not take much revenue to put him in the black. Soon his site will accept advertising and sell a mailing list of customers who are willing to be included.

Mr. Marlin said his superiors at American Express do not know of his venture yet, but that won't last now that he is "going public." He said he hopes they will be supportive.

"I spend 50 hours at American Express and at least 50 hours working on Shoppertunity," he said. "Since I started this in December, I've really devoted my life to it."

After looking at his site, Karen Epper, an analyst at Forrester Research in Cambridge, Mass., said she liked the concept. "Overall, we're not high on the idea of on-line shopping malls," she said. "But it seems like their whole pitch is, 'We're going to be the Good Housekeeping seal of approval for on-line shopping.' As SET creeps onto the scene, that's going to be a bigger and bigger issue."

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