Classified advertisements are a catalogue of our everyday desires. If they reflect a culture's sensibilities, then it is only natural that the ads of the 21st century have migrated online. The most popular online ad site is Craigslist, which grew out of an e-mail list of local events started by San Franciscan Craig Newmark in 1994. The site officially launched in 1995, incorporated in 1999 and today is one of the world's most popular Web sites.

The design of the site is starkly utilitarian, with straightforward lists of links for users to peruse, free of any apparent graphic design or glitter. Best of all, it's free -- free of commercial advertising and free for most users to post classifieds at no cost. The site has no pop-up or banner ads and doesn't even require users to register. The company's revenue comes from the fees it charges employers who list job openings and real estate brokers who list apartments for rent.

Like traditional classified ads, the listings on Craigslist showcase a wildly diverse range of needs and wants. Craigslist includes traditional personal ads, but there is a subcategory for bolder individuals. Called "Casual Encounters," it functions as a virtual parlor for people seeking nearly anonymous, commitment-free sex. This part of the site has recently received attention for facilitating prostitution.

Delving into Craigslist would keep a sociologist (or perhaps a behavioral psychologist) busy for years. Judging by the majority of the postings, what users most want and need is great real estate and lots of sex.

Erotic services aside, Craigslist is something of an anomaly in the dot-com world. By its founder's admission, the company is not interested in maximizing profits. It pours money into a Craigslist Foundation that trains leaders for work in the nonprofit sector. Even its address bar icon -- a purple peace sign -- signals a distinct point of view.

In short, Craigslist is not just a company; it's a philosophy. And that philosophy has many appealing qualities: a resistance to excessive advertising, a sincere commitment to making the minutiae of daily life in the wired world a little easier to manage and a sensibility that places a great deal of trust and optimism in one's fellow human beings. The oft-touted Craigslist mantra is "people helping people."

But online communities face peculiar challenges. Communities, after all, can thrive only when trust is a shared value among members and when violations of trust are not widespread or are effectively penalized. Unlike other online sites such as eBay, Craigslist has no reputation-ranking system to assess the trustworthiness, let alone the identity, of its users. Anyone can pretend to be anyone and sell just about anything.

As a result, when you agree to barter with Miss Kinky Boots, you have no idea if she is an honest broker of services, or even if she is really a she. Since Craigslist is not legally liable for the misrepresentations of those who place ads on the site, and since it has fewer than 30 employees monitoring millions of advertisements, users assume a certain level of risk with every one of their transactions. Examples of fraud on Craigslist abound -- and in a few notorious cases, users have suffered much worse than fraud.

Still, there are clearly many people who take solace in the sense of community they find in their interactions on Craigslist. As Wired noted in a story about the documentary 24 Hours on Craigslist, the filmmakers "interviewed one woman who said she interacts with other posters so much that she feels a greater sense of community on the site than on the neighborhood block where she lives."

Yet for all the talk of Craigslist's aim to be community-centered and "noncommercial," and for all of the site's inclusive, soft-socialist sensibilities, the fact remains that its raison d'être is facilitating commerce, not community. Craigslist is less a genuine community than simply a new kind of counterintuitive brand. It is a hive of micro-capitalism, with people buying and selling from one another -- and scamming one another, too.

"People helping people" is an inspiring motto, to be sure. But until we understand more about how social behavior is altered by the medium of online technology, a better mantra might be caveat emptor.

-- Christine Rosen is a senior editor of The New Atlantis and a fellow at the Ethics and Public Policy Center. E-mail her at crosen@thenewatlantis.com. A longer version of this essay appears at thenewatlantis.com.