craigslist could probably eliminate a ton of spam by simply editting out the urls that go thru it's email system.

i.e. if someone includes <a href> inside the email body, simply unscramble it and show the full link.

And on the bottom of the email spell out a warning about affiliates.

This will eliminate most of the spam that goes out to people who list stuff on craigslist.

To fight spammers that list fake listings to gather email addresses...simply implement a private message system for craigslist. Then you can analyze all emails that get sent to users and do the same trick as #1.

You can also limit exposure by blocking messages based on user IPs. Chances are someone from China listing in New York craigslist is a fake.