You won't make any friends at craigslist (in fact, they'll probably block you), however you could try "importing" craigslist ads (of course, credit and link properly). I don't know if I'd suggest it, but it's a route you could consider. The founder of Lyrics Wiki (
http://lyrics.wikia.com/ ) indexed other lyrics sites, and eventually was bought by Wikia.
Also, look at a few of the paid craigslist alternatives- they might be willing to give you listings if you send traffic their way.
I asked this same question to Jimmy Whales a few months ago. Wikipedia started as a paid service, where professionals were paid for their work. It never got big, so they changed their business model. For them, the already-made articles were enough of a jumping point.
Side note: craigslist already has many, many more-featured competitors. Do you have a theory as to why craigslist is still the most popular software? Make sure you do before you bother creating a craigslist competitor.