"People posting ads on Craigslist do not necessarily want or intend for it to be reposted on other sites."
That argument appears to be invalidated by Craiglist's own terms of use, which say that when you upload a listing to Craigslist they can syndicate it wherever they want (http://news.ycombinator.com/item?id=4287519).
Well, that's a fair point, but I don't think that right matters much unless and until they exercise it. A better argument (against myself) is that they're apparently willing to sell some type of access to your posts for use on mobile apps.
Still, I think there's something admirable in the simplicity and transparency of interacting with Craigslist. What you see is pretty much exactly what you get.
The key point is that the TOS says that you grant Craigslist the right to redistribute your listing where ever they see fit, it doesn't say some third party entity has the right to do that.
As far as I'm aware, if you post information in a place where it will be publicly accessible, then you are tacitly agreeing that third parties will be able to access it and use it as they please. That's not a right that needs to be given to these third parties. It exists from the get-go, and must be explicitly taken away by something like copyright.
So I can legally start amazon-copied-reviews.com and scrape every product review from amazon.com with my own referrer links to Walmart without fear of repercussion? Sweet.
That argument appears to be invalidated by Craiglist's own terms of use, which say that when you upload a listing to Craigslist they can syndicate it wherever they want (http://news.ycombinator.com/item?id=4287519).