Note to would-be netrepreneurs: if you want to avoid being sued, don’t do anything (the Craigslist way). If you are simply a passive conduit, it is quite possible that you won’t be held responsible for the nefarious and illegal (and perhaps more importantly, profitable) activity of your patrons on the grounds that you’re not actually inducing the conduct. At least that’s what a federal court seems to have held in Dart v. Craigslist, Inc.
The fact that Craigslist also provides a wordsearch function does not change the analysis. The word-search function is a “neutral tool” that permits users to search for terms that they select in ads created by other users. It does not cause or induce anyone to create, post, or search for illegal content.
Similar analysis has sometimes prevailed in the Copyright context, those wily content owners are getting more and more desperate. Desperate enough to tell a shop assistant in England that she could not sing while she stacked shelves without a performance license. But maybe Copyright over-enforcement can be a force for good, like its use by a group of musicians who want to use their Copyright pull to stop Guantanamo interrogation with their music.