Saying you have something doesn't really mean you have something - I'm not sure that would stand in court. IANAL, of course, and anything can happen, especially in bizarro world of computerized information where there's no legal practice refined by hundreds of years of common sense applications, so such thing may probably be useful for Craigslist to go after some content scrappers. But I seriously doubt they would ever go after the post owner for reporting his own content.