Craigslist classified ads may have kneecapped the newspaper business, but they are doing wonders for amateur comedians, finders of false teeth, and fashioners of crucifix-shaped guitars played by men dressed as Funky Jesus.

"I am an atheist seeking friends who are pious and who will be wisked away for the rapture that kirk cameron says is coming may 22nd 2011," said one obviously humorous ad last year. "I get to keep all your stuff because you won't need them as you will be in rapturous bliss basking in the love of the great creator. I, however, will need your stuff since I will be trapped here on earth with all the rest of the unclean, deniers, and cravens."

But most unbelievable posts concern sex. A famous find from 2008 instructed any interested woman to arrive at a certain hotel, walk into a certain room (the door would be open), and lie down naked on the bed with posterior in the air. The woman was to pick up the NES controller lying there and start up Super Mario Bros. The man who posted the ad—hiding in the bathroom until this point—said he would only come out when the game had begun.

"When you reach the end of level one, make sure to trigger the fireworks. This is vital to the entire experience. I must hear the fireworks. When level 2 begins and Mario walks into the pipe, I will penetrate you... I will continue having sex until the level ends. DO NOT take the secret level skip. If you die I will pull out and spank you until the level restarts."

And the trolls aren't just writing the posts; at least one guy makes his living by responding to classified ads in the most obnoxious possible way and posting the results.

So when one sees an ad on the site asking for "any single moms and dads around that would like to see me use your daughters panties" to masturbate with, one hopes that it falls into the same "unbelievable" category as many other posts. But what happens when the thirtysomething man writing the ad is serious?

"im DEF interested"

Baltimore County Police Detective Kevin Smith has served as a cop since 1994, starting on street patrol but taking classes in online investigation and peer-to-peer networking until, in 2008, he was assigned to the department's Crimes Against Children Unit. As part of the job, he struck up conversations with local people whose online postings suggested an unhealthy sexual interest in children.

On September 17, 2010, Smith responded to a Craigslist post titled "Your Daughters Panties - m4w - 30 (Harford County)." He told the person behind the post that he was a "40 wm [white male] stepdad with 11 yo step daughter... we r close and like to play... if interested let me no... we can work something out for her panties."

"I wasn't even really looking for panties, i was looking for someone like you to reply."

The response came quickly. That evening, lordbdub@yahoo.com wrote back with the name "Billy Warner" in the header of his e-mail. "Hey man im def interested," he told the detective, "and if you have her trained already im DEF more interested in that then in just the panties."

So—not a joke, a prank, or a shock post. This was the real deal, and Billy Warner was insistent. He sent an e-mail at 6:14pm, then another at 9:55pm, and finally a third just after midnight. "Hey man are you around or what??" he asked. "I want to get some info about your step:)"

The e-mails continued throughout the weekend. On Sunday evening, Warner wrote twice more, begging for pictures or even just descriptions of the girl. "Is mommy in on it too?" he asked.

The next week, Smith began exchanging e-mails with the man. It took only hours for Warner to send over two pictures of nude children, one of which probably qualified as child pornography.

Conversations moved onto Yahoo Instant Messenger, where Warner made clear that his ad had only been a dangler, that he hoped to find something less legal than a child's underwear. "I wasnt even really looking for panties," he said. "i was looking for someone like you to reply."

People like Detective Smith had been replying—too many, in fact—and Warner freaked. Two days after he first exchanged instant messages with Smith, Warner returned to say that someone else had contacted him online with a very similar persona to Smith's own adopted "40 year old stepfather."

"I deleted my whole hard drive cuz i thought you 2 were the same guy and were coming for me," Warner wrote when he got back in touch. But his groin overpowered his good sense, and he went back to communicating with Smith—even providing his home phone number on October 4. Smith called him; "Billy" answered. For someone with his proclivities, Warner wasn't exercising much caution.

This isn't unusual. As Steve DeBrota, an assistant US attorney in Indianapolis who has brought down major child porn sites through extensive litigation, told me recently, "Being able to find a like-minded individual easily and communicate about sexual fetishes wasn’t very easy to do in 1992, but today is trivially easy. That lowers a barrier to what are people with already low impulse control.”

Now Smith just had to find Billy Warner.

Listing image by Photo illustration by Aurich Lawson