There's plenty you could do to make a better Craigslist, simply because they're so slow to roll out any new features. As a result, there's a large number of websites and mobile apps built to augment the basic functionality of vanilla Craigslist.

But I highly doubt you could have any success grabbing market share, because none of these functional shortcomings are enough of a problem to the Average User that they'd be interested in using a service with no reputation. Not to mention the network effect.