Thanks to Craigslist, law enforcement agencies are now battling drug dealers in cyberspace.   Each month, more than 50 million free classified ads are posted on one of the Craigslist websites, which serve 570 cities in 50 countries.  Unlike ads in newspapers and magazines that are reviewed by editors, Craigslist ads are for the most part not checked for content.  This allows drug dealers to post ads for both illicit and prescription drugs.   In cities and towns across the U.S., narcotics investigators are making arrests by responding to ads that are publicly displayed on Craigslist websites.…