If you had posted the ad as a non-registered user, Craigslist would have sent an email to your email account asking you to verify the ad. Search for the email from Craigslist in your email inbox and open it.
Inside the email, there will be a link for cancelling your ad. Click on the link. The destination web page will have a ‘Delete this posting’ button on it. Click the button to cancel the Craigslist posting.
However, if you are a registered user on Craigslist, cancelling a Craigslist posting does not require you to locate a particular email amongst thousands of emails in the inbox. With your Craigslist username and password, login to the ad posting service at Craigslist by clicking on the ‘My Account’ button in the top left corner of the Craigslist home page.
A list of all the ads you posted after signing into the as posting service will be available on your account page. If you have posted lots of ads recently, search for the ones that relate to products or services you have already sold. Next to each ad, there will be a ‘Delete’ button. Simply click on it for all the ads that you want to remove.