The Craigslist.org classified service offers controls that limit how frequently a post may be placed and makes a number of internal determinations to decide whether or not the post is a "spam" advertisement. When a post is determined to be untrustworthy it is "ghosted" by craigslist, which means the system accepts the post but does not publish it to the main page. The internal calculations Craigslist makes in this determination are not available to the public but it is possible to determine if the article has been ghosted by doing a simple check.
Post your Craigslist ad as usual in the city and section of your choice.
Check your email and verify that the ad request has been received. If asked to click a link to confirm the post be sure to do so and click "Publish."
Wait at least fifteen minutes for the article to post. This may sometimes take longer so you may wish to wait up to an hour.
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Visit the section you posted your ad in and look for it in the list. If it is not present and you have not received an email stating it has been flagged for violation of the Craigslist terms of service your ad has likely been ghosted.