Like any other website that allows users to post content, Craigslist has a set of rules that define allowable content. Because Craigslist employs only about 30 people to manage its sites worldwide, it's unusual that a Craigslist employee would block your ad. Instead, users have the ability to flag ads they believe are a violation of Craigslist policies. If enough people flag your ad for any reason, it's blocked from the website.
Posting in the wrong category can be a fast track to getting your ad flagged and removed from Craigslist. Most of the sections and categories should make sense if you take a close look at the options. For example, if you are selling video games, the Video Games category of the For Sale section is the place to put it. If you are looking for volunteers for a weekend project, this should be placed in the Volunteer category of the Community section, not the jobs section -- unless you're paying people to volunteer. If you're uncertain where to place your ad, take a look at other ads in the most likely category to see if they are similar to yours or not.
Spam or Spam-like Ads
Spam is seldom tolerated in Craigslist sections, but even if you see someone else doing it, don't assume that your ads won't be blocked. Posting the same ad in different categories, or posting an ad within 48 hours within the same category both count as spam. Posting ads in more than one city counts as spam, too. Remember that Craigslist is essentially a community-based website. Posting any ad that isn't local to your community can count as spam -- for example, directing people to a website to buy something or to promote your latest YouTube video.
Violating Craigslist's Policies
Violating Craigslist's policies will almost always get your ad blocked. For example, ads promoting or discussing illegal activities violate these policies, as do ads containing copyright violations, or ads that threaten or harass others. Craigslist prohibits ads related to the sale of firearms, tobacco, alcohol and pets. Offers of counterfeit or replica items, lottery or raffle tickets, prescription drugs or ID cards are also prohibited. Affiliate marketing, pyramid schemes or ads designed to promote website traffic aren't allowed on Craigslist either. (See Resources for the entire list.)
Since Craigslist relies on its users to flag ads, there is a possibility that your ad may be blocked for no reasons you can identify. For example, if you post an ad that is similar to an ad that was recently flagged and removed, people may mistakenly believe that your ad was by the same person. If you inadvertently use a phrase in your ad that people believe refers to something prohibited, your ad may be flagged and removed from the website.