That's the conclusion of Monday's Gibson v. Craigslist, Inc. (S.D.N.Y.), applying the service provider immunity of 47 U.S.C. § 230: Craigslist isn't liable for the ad, even if the ad itself is tortious:
Plaintiff seeks to hold [Craigslist] liable for its alleged failure to block, screen, or otherwise prevent the dissemination of a third party's content, i.e., the gun advertisement in question, alleging, among other things, that Defendant "failed to monitor, regulate, properly maintain and police the merchandise being bought and sold on its ... website" and "is either unable or unwilling to allocate the necessary resources to monitor, police, maintain and properly supervise the good and services sold on its ... website." It is clear that Plaintiff's claims are directed toward Craigslist as a "publisher" of third party content and "Section 230 specifically proscribes liability in such circumstances."
I don't think that Craigslist should be liable even under normal state tort law: I don't think it's negligent for Craigslist not to police its ads, given the high costs of reading each ad and then trying to figure out whether it offers an illegal transaction or is likely to lead to injury. What's more, while I couldn't find a quote of the ad itself -- if any of you can point me to it, I'd love to see it -- the plaintiff's own motion (p. 7) says that "the content of the ad placed upon defendant's website was not, in itself, objectionable," and that it didn't say things like "'Illegal handguns for sale' or 'Shoot your neighbor.'" The plaintiff's argument therefore seems to be not just that Craigslist should look at every ad (or perhaps at all ads that contain certain keywords), but that it should further investigate the circumstances of the ad to see whether the proposed transaction is illegal (or poses unreasonable dangers) even when the illegality or unreasonable danger is not obvious from the ad's face.
But negligence law is quite mushy and unpredictable in many ways, and even if it doesn't lead to liability, it can lead to a long and very expensive legal fight. 47 U.S.C. § 230, as interpreted by courts, has been considerably more clear and protective, and can often be used to throw out lawsuits very early in the process. So I'm particularly happy (but not at all surprised, given the text of the statute and past caselaw interpreting it) that the court decided the case on § 230 grounds.
Special bonus from reading the plaintiff's opposition to the motion to dismiss (p. 3): "[Craigslist's] only concern is the bottom line, the public be darned." Never quite heard that way of putting it (though I can see why the lawyer didn't want to say "damned").
The next sentence, on the other hand, is less amusing: "For public policy concerns, [Craigslist] must be immediately regulated or shut down." It's also probably a poor way of arguing the case, given that the whole premise of 47 U.S.C. § 230 is that service providers shouldn't be "regulated" or "shut down" by fear of liability; if you're arguing that § 230 doesn't apply, you should probably argue that you're calling for a very modest and narrow sort of liability -- for instance, the liability imposed on newspapers, which one would hardly call "regulat[ing]" or "shut[ting] down" the newspaper -- rather than for regulation or shutdown.
Thanks to Prof. Michael Krauss for the pointer. As I noted above, if any of you can point me to the text of the ad, which I couldn't find in any of the Pacer-accessible documents (the Complaint, for some reason, isn't on Pacer), that would be great -- it's not relevant under the court's 47 U.S.C. § 230 approach, but it might nonetheless be helpful for discussing the broader issues.