He added: "I am fully convinced that Craigslist wants to stop this activity as much as we do."
Buckmaster said the agreement does not cover Craigslist's personal ads, where prostitutes have been found advertising for "dates." But he said the San Francisco-based company has been working with authorities on that issue and on cutting down on the sale of stolen merchandise on its sites.
"We are experimenting with telephone verification in those sections," he said. "We don't have any plans to use credit card verification in that section currently. But this partnership is going to be active in that area as well, anywhere where crime does or could occur."
The agreement was joined by the National Center for Missing and Exploited Children.
"The criminals engaged in the sexual trafficking of children no longer parade them on the streets of America's cities," said the center's chief executive, Ernie Allen. "Today, they market them via the Internet."
The states that signed the agreement are Arizona, Arkansas, Colorado, Connecticut, Delaware, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Mississippi, Montana, Nebraska, Nevada, New Hampshire, New Mexico, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin and Wyoming. Washington, D.C., the U.S. Virgin Islands and Guam also joined.