The Craigslist app by Lifelike Apps, Inc is a great app for both viewing and posting on Craigslist.
There is a free version you can try. You can then upgrade via in-app purchase or buy the full version (which also has some in-app upgrades). But you won't need them to post adds. They just affect how far away you can search (100mi, 250mi, 500mi) when in auto/local mode.