If you've ever used Craigslist.org-- a fantastic and rather odd resource-- you may have noticed that it's heavily biased towards per-city searches. If you want to search for something nationwide, you can't do that using Craigslist. But you can do it using this page, which performs the same search across all Craigslist.org cities (in the USA), and shows the results as a single page.
Jeff, Your search tool just appeared on our radar and while we appreciate your desire to extend our search functionality it goes against the basic intent of Craigslist to be a local tool. Also it's in direct violation of our Terms of Use, section 5: Additionally, you agree not to: use automated means, including spiders, robots, crawlers, data mining tools, or the like to download data from the Service - exception is made for internet search engines (e.g. Google) and non-commercial public archives (e.g. archive.org) that comply with our robots.txt file; Please disable your tool for public use. If you refuse to do so we'll be forced to block all the proxy servers that you're using. -Bill -- Bill SwingleSystems Administrator
Further information can be found on this craigslist.org forum thread.