There are lots of reasons why Craigslist might have banned you. Here are just a few:
You attempted to sell or solicit something on the site’s prohibited list
Your post contained offensive or obscene language
Other users flagged your post
You posted too often in a short space of time
Craigslist noticed you putting up the same post on numerous regional boards
Your post was spammy, misleading, or intended to direct users to a second website
You attempted to scrape the Craigslist website using automated tools