I've analyzed this market quite a bit (as I've thought about creating a startup in this area) and the best attack vector I identified was to commodotize the platform.

Do to Craigslist what slinkset does to reddit. Go after a thousand niche markets. Craigslist lets you suggest new locations but that's about it. But there's no reason every small town/organization/hobby/school/company shouldn't be able to have it's own classifieds website.