The problem with any scraping solution of craigslist is that they automatically block any IP address that accesses them 'too much' - which usually means more than a few hundred times a day. So as soon as your tool got any kind of popularity, it would be shut down.
That's why the only craigslist search sites that have lasted either use frames (like searchtempest.com and crazedlist.org) or google (like allofcraigs.com).
What 3taps does is to gather craigslist listing from third party sources 'in the wild' - things like the Google and Bing caches for example.
Edit: this answer is no longer up to date. Most classifieds search engines that include results from craigslist now use Google Custom Search or similar solutions from Yahoo or Bing. SearchTempest uses both. Allofcraigs is now adhuntr and uses Google. Crazedlist has shut down.
