All well and good, except that it would result in my server quickly being blocked by craigslist. This has happened to many other sites in the past, and will continue happening to new sites as soon as they gain enough popularity to get noticed.
I know it's not technically difficult. The issue is that craigslist doesn't allow it. (More details in the FAQ
if you're interested.)
That said, I'm working on an alternate search mode using a google custom search that will return all the results in one window. The results are delayed slightly and are not sorted by city, so it's not ideal for people searching small regions; however, for those searching the entire country for rare items it should be quite useful.