What AirBnB did was a little spammy (and probably violated CL's ToS). Same goes for ODesk and other freelance boards (I got one from HireTheWorld just the other day).
When someone wants all your personal information or help moving (i.e. "laundering") money overseas, that's an entirely different animal.
Lots of companies use affiliates to do the dirty work for them, then ban affiliates that get caught. New affiliates pop up, or the banned affiliates create new accounts and start over. That leaves sites like Craigslist in the unenviable position of playing whack-a-mole to try and ban spammers.
Other variations on these include fake profiles of women on (insert social networking site her) who inexplicably "friend" men and entice them to sign up for dating/adult sites where the target can see more photos, and gambling instruction guides/beat the casino secrets uploaded to torrent sites for free. Naturally affiliate links to lots of online gambling sites are embedded in the guides.
I guess the takeaways are:
1. Never trust user generated content.
2. Whenever there's a way for people to produce content (effectively) for free, the affiliates/scammers will be there. I built a rental-listing site a few years back. Not long after I stared getting real listings, I started getting rental scams. I had almost zero traffic, but the tiny blip on the radar screen my site produced drew scammers like flies.
4. It's often better to ask forgiveness than permission. What repercussions did AirBnB suffer for their posts? What real repercussions do any of the spammers face?
5. Just writing about affiliate marketing, well, at least this side of it, makes me feel icky.
It's worth pointing out, that if you used some sort of standard back end like Wordpress or some other user content system, there are bots built to search the web for those and post to them automatically. I wouldn't put it past someone to throw in a check for if it is real estate related and handle slightly more advanced back ends like Drupal.
I did have a Pligg site I put up as a quick project and that was a victim of the automated searches for the "submit" page of a Pligg site. Even adding a CAPTCHA didn't help, so I assume offshore workers were adding links manually.
I definitely consider them different, though perhaps I didn't highlight that difference sufficiently.
I made a modification and titled the section as such, with a credit to you in the post.