While hardcore Craigslist.org community members may find this post disturbing, I am writing it because I think it points out flaws in the system and needs to be corrected to flush out the spammers and make it a valuable classifieds service.
Spammers have been using Craigslist since they have heard how many unique visitors go there every day. It used to be that anyone can post without having an account.
What happened was that you can create a default email address in your cpanel so no matter what address on your domain (even fake) that you use to receive email, it will instantly be forwarded to your selected email address that is real. Combine that with people who own multiple domains.
It is pretty easy to post the same ad in different cities and categories. You can even upload the ad image to another domain or use a 301 redirect on a URL to get around spam filters.
A recent change made it so that you have to have an account, and a phone number has to be associated with that account before you can post in most categories.
This cut down on allot of spam at first. But spammers know that you can get around this issue by creating multiple accounts with multiple domain email addresses. Then the simply sign up for a ghost number with Vonage that forwards to your main number. It only cost $10 per month for an extra number with Vonage. You can use the number to verify the account on Craigslist then cancel the number with Vonage. While not all vonage numbers cannot be verified, most of them can.
I think that the way to truly get at killing the spam is to develop a script that checks for the same ad text, then verifies that the url in the ad is forwarded to another ad with the same text. On top of that they can do the same thing with image filenames and filesizes, then run the check as before with the redirect the link is pointing to.
While the account thing may have worked a little in the short term, it only lasted maybe a week and turned off other regular craigslist users that have Vonage numbers that could not be verified. It was a bad business move in my opinion when there are other technical solutions to the spam problem.