There's a really good paper here (pdf) on how spammers attack search sites, including craigslist, in order to get high search ratings and to get money from unintended click-throughs.
Spammers use questionable search engine optimization (SEO) techniques to promote their spam links into top search results. In this paper, we focus on one prevalent SEO technique, redirection, where one can identify spam pages by the third-party domains that these pages redirect traffic to. We propose a five-layer, double-funnel model for describing end-to-end redirection spam, and identify prominent domains on each layer using two sets of commercial keywords: one targeting spammers and the other targeting advertisers. The methodology and findings are useful for search engines to strengthen their ranking algorithms against spam, for legitimate website owners to locate and remove spam doorway pages, and for legitimate advertisers to identify unscrupulous syndicators who serve ads on spam pages.
This really affects us at craigslist, we put a lot of energy into fighting spammers of all kind.
Kudos to the investigators: Yi-Min Wang (Microsoft Research), Ming Ma (Microsoft Research), Yuan Niu (UC Davis), Hao Chen (UC Davis)