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Craigslist is an automated system in that for the most part it does not rely on human moderators.

Instead it relies on community moderation, or in other terms, flagging.  An ad may be flagged as miscategorized, prohibited, or spam/overpost.

The Craigslist user can flag an ad for any reason and the three types of flags all count the same.  The spirit of the Craigslist flagging system is one flag per person per ad.  This is enforced by IP address.

You can flag an ad many times, but if you have not somehow changed your IP address only one flag counts.

How can an ad be flagged more than once and have it count?

The simplest way is to flag it at work and then flag the same ad at home.  Work and home IP addresses will most certainly be different.  Stop off at the mall on the way home and use public hot spots.

I can use my neighbor's unsecured wireless router.  In fact, I can see several unsecured networks right now.  My DSL package comes with more than one IP address.  They count too.  But there is a much better way to flag an ad multiple times and have each flag count, and this is by the use of proxy servers.

First we must talk a little bit about flag thresholds.

Flag Thresholds

Every ad has a flag threshold.

This is the number of flags it takes from different IP addresses to remove the ad from the system.  All Craigslist staff will admit to is that the flag threshold is between 2 and 10,000 for any ad.

The exact number of flags it takes can vary by city, list, and account.  An account that has a history of getting flagged down may tend to have posted ads with low thresholds.  However an account is not needed to post an ad.

Posts made without a user account may tend to be low as well.  In periods of heavy spam, which seems to be always, Craigslist may lower the flag thresholds.

Proxy Flagging

Proxy servers hide your IP address and Craigslist only sees the IP address of the proxy.

Anyone can use proxies to flag ads more than once and have the flags count.  Enough proxy servers at my disposal ensures that I can always flag an ad down.

I usually have about 50 proxies bookmarked.  I also have ways to easily find more when needed.  Proxy bookmarks do expire and need to be replaced.

Most of my proxy flagging takes place on one list in one city.  Sometimes, I venture out into other lists in my city and other cities.

In over a year of proxy flagging I have never seen an ad that I could not take down or at least cause it to be ghosted.  (Ghosting occurs when the ad has been removed from the master index, but a direct URL to the ad still works.)

I have never seen a threshold greater than 75 with the possible exception of the personals which I consider to be a wasteland anyway and almost never go there.

Of course, I have no idea what other people are doing in terms of flagging, so I cannot tell for sure.  I am in a major city and my list of interest is fairly high volume.  I expected the thresholds to be higher, but they were not.

When I flag ads in other major cities I never find high thresholds.

Thresholds tend to be low because people generally do not bother to flag.

I see evidence of this every day.  I often go back a day or so in a list and find spam that probably went live with a threshold of 1 or 2.  I flag it once and down it goes.

Sure, it may have gone live with a normal threshold and my flag just happened to be the one that took it down, but I see it happen so often that I have to conclude it is because people don't flag.  The lack of flagging on Craigslist is much like a democracy with poor voter turnout.  Many bad ads on Craigslist simply do not get flagged enough to be removed.

List after list in city after city are full of scams, spam, rip offs, and terms of use violations.

Craigslist could attempt to block proxies, but they don't.

I speculate this is because proxy servers come and go fast.  It would be difficult to try to keep track of them all.  New ones will always be popping up.

Craigslist wants more flagging.  Craigslist is probably blocked at many work places and the only way for people to get to Craigslist at work is through a proxy.  When these people flag, Craigslist wants it to count.

It is also very possible that Craigslist has concluded that proxy flagging does more good than harm, that is to say an individual flagging more than once helps to take down bad ads more often than not.

Finding Proxies

Of course, Yandex/Google is your good buddy.

Search for "proxy servers".

Here is a link to a proxy server that in turn provides access to a small network of proxy servers: demo1.proxysplit.com

Proxy servers are pretty easy to find.

Risks

I cannot swear that every proxy server is safe.

Some may be the tools of spammers looking to take over machines and turn them into spam bots.  So before going crazy with proxy servers make sure Windows is up to date with security patches and virus/spyware updates.

A better choice would be to use Linux or Mac OS X for proxy flagging.