From Craigslist Terms of use http://www.craigslist.org/about/terms.of.use
a. Postings
craigslist is intended and designed as a local service. A user may post content only to the single specific geographic area offered on craigslist (see http://www.craigslist.org/about/sites) for which that content is most relevant. The same or substantially similar content (for example, an ad for a particular item or service, a particular offer, a particular message or a particular comment) may not be posted to or communicated via more than one such geographic area. Content that is equally relevant to multiple (i.e., more than one) geographic areas should not be posted on craigslist.
And more...
Users may not circumvent any technological measure implemented by CL to restrict the manner in which content may be posted on craigslist or to regulate the manner in which content (including but not limited to email) may be transmitted to other users. This prohibition includes, without limitation, a ban on the use of multiple email addresses (created via an email address generator or otherwise); the use of multiple IP addresses (via proxy servers, modem toggling, or otherwise); CAPTCHA circumvention, automation or outsourcing; multiple and/or fraudulent craigslist accounts, including phone-verified accounts; URL shortening, obfuscation or redirection; use of multiple phone lines or phone forwarding for verification; and content obfuscation via HTML techniques, printing text on images, inserting random text or content "spinning."