Craigslist Mobile is a Craigslist app for your iPhone that has all the bases covered.
Although it's not great to look at and feels a little cluttered, this app really does do everything you could want. You can buy or sell with the app. You also have access to your Craigslist account and can apply multiple filters to your searches so you can be as accurate as possible. There's also a search agent, so the app will continue to search even after you've left the app.