All you can really do isjust "flag" the ad when you click on it. It gives you the ability to delete the posting. Nothing more. Craigslist gives no way's to post negative reviews. Unless you take the ad posting and info and post your own ad as the negative outcome or info. To pass on your experience. Lots of others are doing it as scam alerts or fraud alerts. Only remember that if you do, then the person whom your referring to also can "flag" your posting also.

As far as I know, you will have to post an ad listing the name of the person or business you bought it from and the negative experience you had.