NEW YORK » The Craigslist ad offered Percocet pills for sale, but warned: “No LE please.” Meaning: No law enforcement.

Like that made a difference.

The 40-year-old man accused of placing the ad was among 21 people arrested in an attempt by the New York Police Department to make an example out of some of the smallest of small-time drug dealers: students, young professionals and others who clean out the medicine cabinet and then are brazen enough — and foolish enough — to offer the pills for up to $20 a pop over the Internet.

“Whether the drug deal occurs on the street corner or on the Internet, it’s a crime,” Bridget Brennan, special narcotics prosecutor for New York City, said today in a statement announcing the arrests.

Undercover narcotics investigators answered the ads and ended up buying handfuls of powerful prescription painkillers and other pills for a few hundred dollars, typically in broad daylight and in public settings such as coffee shops, Penn Station or Washington Square Park in Greenwich Village.

Some of the sellers turned out to be run-of-the mill drug dealers also peddling cocaine and heroin, police said. But many were more mainstream: Among those arrested were a New York University graduate student, a financial adviser and a 62-year-old woman who works as a freelance photographer.

The pills came from the sellers’ own meds or were stolen from relatives, friends and co-workers, authorities said. Some of the dealers were out to make a quick buck — even though their backgrounds would suggest they didn’t need the money.

The arrests come as law enforcement agencies around the country battle a surge in illegal sales of highly addictive painkillers like Percocet and Roxycodone and, increasingly, attention-deficit drugs like Adderall — transactions that now rival the cocaine and heroin trade both in volume and as a public health hazard.

Because the drugs have legal medical uses, they carry less of a stigma than illegal narcotics. After they were arrested, some of the sellers claimed they didn’t know what they were doing was a crime. But investigators don’t buy it.

“You’d have to be living under a rock to not know it’s illegal,” Brennan said.

Brennan’s office reached out to San Francisco-based Craigslist earlier this week to try to get its cooperation in finding ways to discourage the Internet marketing. A Craigslist spokeswoman had no comment today.

The undercover investigators began answering Craigslist ads late last year. By the time they were done, they had made 63 buys — about $19,000 in pills and $10,400 in cocaine.

One ad was placed by a self-described “friendly NYU student.” It offered “pain and anxiety relief.” Just in case there was any confusion, it signed off with “perc roxy.” And a smiley face.

A criminal complaint accused Anthony Vargas — the Manhattan man who politely asked police not to respond to his ad — of meeting the same undercover officer four times over the summer near Union Square to sell dozens of oxycodone pills. Vargas, 40, was sentenced to nine months in jail on Wednesday after pleading guilty to criminal sale of a controlled substance, prosecutors said.

Despite the enforcement effort, the advertising continued unabated today.

One posting from someone in New York City read: “Hello ladies and gentleman. If your in pain and need assistance look no further. I’ve got 30 (mg) ways to help!! … Not affiliated with any sort of law enforcement? If ALL of these pertain to you, email me with what your looking for (quantity) and we will work it out.”