Can a web site be liable for things its users post? No, the federal 47 U.S.C. § 230 statute generally provides, whether the posts are libelous, emotionally distressing, crime-promoting, or whatever else (with some exceptions, such as for federal intellectual property laws). But the sites are responsible for their own speech, and their own decisions that themselves create actionable speech.

To give an example from one leading case, the Ninth Circuit Roommates.com decision:

A web site that features ads for roommates isn't liable when users themselves post ads that contain discriminatory roommate preferences (even if the law can bar such discrimination in roommate selection, which is a separate question). That's just the users' own choice, for which they and only they can be held liable.

But the site can be liable if it expressly requires subscribers to state such preferences, since then it is itself "develop[ing]" the legally punishable material.

Now say that a web site lets users post ads for gun transactions. Under § 230, it can't be liable simply because some ad that users posted leads to an illegal transaction. But can it be liable on the theory that some of its design features, such as the ability to search for ads by private sellers, especially facilitate crimes? (Under federal law, private sellers, unlike professional gun dealers, don't have to do a background check on their buyers; ads from such sellers might thus be especially useful to felons and others who would fail such a check.)

In Daniel v. Armlist, plaintiffs, the Wisconsin Court of Appeals, and one Justice of the Wisconsin Supreme Court (Ann Walsh Bradley), said the site could indeed be liable for this. But the majority disagreed, and said there was no liability. First, the facts and the plaintiff's theory:

[Plaintiff Yasmeen] Daniel's tort action [for negligence, negligent infliction of emotional distress, civil conspiracy, aiding and abetting tortious conduct, public nuisance, and wrongful death] arose from a mass shooting in a Brookfield, Wisconsin spa that killed four people, including Daniel's mother Zina Daniel Haughton. Daniel alleged that the shooter, Radcliffe Haughton, illegally purchased the firearm after responding to private seller Devin Linn's post on Armslist's firearm advertising website, armslist.com…. We disagree …. [Title] 47 U.S.C. § 230(c)(1) prohibits claims that treat Armslist, an interactive computer service provider, as the publisher or speaker of information posted by a third party on its website. Because all of Daniel's claims for relief require Armslist to be treated as the publisher or speaker of information posted by third parties on armslist.com, her claims are barred by § 230(c)(1)…. Armslist.com is a classified advertising website similar to Craigslist. Prospective sellers may post advertisements for firearms and firearm-related products they wish to sell, prospective buyers may post "want advertisements" describing the firearms they wish to buy. Buyers and sellers may contact one another either through personal contact information they provide on the website, or by using armslist.com's "contact" tool…. [T]here is no allegation that Armslist itself participates in the purchase and sale of firearms beyond allowing users to post and view advertisements and contact information on armslist.com. According to Daniel's allegations, Radcliffe shopped for the murder weapon exclusively on armslist.com because he recognized that the website's design features made it easier for prohibited purchasers like him to illegally purchase firearms. Armslist.com allows potential buyers to use a "seller" search filter to specify that they want to buy firearms only from private sellers, rather than from federally licensed dealers. Private sellers, as opposed to federally licensed gun dealers, are not required to conduct background checks in Wisconsin. The website also does not require buyers or sellers to create accounts, which encourages anonymity, and displays next to each advertisement whether the account is registered or unregistered. Armslist.com allows users to flag content for a number of different reasons, including "scam," "miscategorized," and "overpriced," and uses these flags to delete certain posts. However, it does not allow users to flag content as "criminal" or "illegal" and does not take action to delete illegal content. The website contains no restrictions on who may create an account, or who may view or publish firearm advertisements using its website. The website's lack of restrictions allows buyers to avoid state-mandated waiting periods and other requirements. Armslist does not provide private sellers with legal guidance as to federal and state laws governing the sale of firearms. Daniel's complaint also suggests several simple measures Armslist could have taken in order to reduce the known risk of illegal firearm sales to dangerous prohibited purchasers. Daniel alleges that Armslist could have required buyers to create accounts and provide information such as their name, address, and phone number. In states similar to Wisconsin, where there is online access to an individual's criminal history, Armslist could have required potential buyers to upload their criminal history before their accounts were approved. She alleges Armslist could have allowed users to flag potentially illegal firearm sales. It could have prohibited users from obtaining one another's contact information until Armslist confirmed their legal eligibility to buy and sell firearms. According to the complaint, all these measures would have reduced the risk of firearm sales to persons prohibited from owning a firearm. Based on all these features and omissions, Daniel's complaint alleges that Armslist knew or should have known that its website would put firearms in the hands of dangerous, prohibited purchasers, and that Armslist specifically designed its website to facilitate illegal transactions….

The court reviewed various § 230 precedents from other jurisdictions, and concluded that Armslist was immune from liability. Under § 230, the court held, Armslist could only be liable under state law for posts on its site to the extent that it actually "develop[ed] the content" of the posts by creating them or at least "materially contribut[ing]" to their illegality—and Armslist did not do so:

The concept of "neutral tools" provides a helpful analytical framework for figuring out whether a website's design features materially contribute to the unlawfulness of third-party content. A "neutral tool" in the CDA context is a feature provided by an interactive computer service provider that can "be utilized for proper or improper purposes." A defendant who provides a neutral tool that is subsequently used by a third party to create unlawful content will generally not be considered to have contributed to the content's unlawfulness.

Examples of such neutral tools include a blank text box for users to describe what they are looking for in a roommate, a rating system that allows consumers to award businesses between one and five stars and write reviews, and a social media website that allows groups to create profile pages and invite members. All of these features can be used for lawful purposes, so the CDA immunizes interactive computer service providers from liability when these neutral tools are used for unlawful purposes. This is true even when an interactive computer service provider knows, or should know, that its neutral tools are being used for illegal purposes…. [T]he difference between a neutral design feature and the development of unlawful content is the potential for lawful use…. [I]f a website's design features can be used for lawful purposes, the CDA immunizes the website operator from liability when third parties use them for unlawful purposes. … Daniel's argument is based primarily on the assertion that Armslist's design features make it easier for prohibited purchasers to illegally obtain firearms. She asserts that Armslist should have known, actually knew, or even intended that its website would facilitate illegal firearm sales to dangerous persons…. [But under § 230(c)(1),] the issue is not whether Armslist knew, or should have known, that its site would be used by third parties for illegal purposes. Instead, the issue is whether Armslist was an information content provider [and not solely a platform provider] with respect to Linn's advertisement. Armslist.com's provision of an advertising forum and the related search functions are all "neutral tools" that can be used for lawful purposes. Sales of firearms by private sellers are lawful in Wisconsin. Further, private sellers in Wisconsin are not required to conduct background checks, and private sales are not subject to any mandatory waiting period. Accordingly, the option to search for offers from private sellers is a tool that may be used for lawful purposes. The remainder of the design features referenced in Daniel's complaint—lack of a "flag" option for illegal activity, failing to require users to create an account, failure to create restrictions on who may post or view advertisements, and failing to provide sufficient legal guidance to sellers—are voluntary precautions that the CDA permits but does not require. Whether or not Armslist knew illegal content was being posted on its site, it did not materially contribute to the content's illegality. Daniel attempts to evade the CDA by asserting that creators of armslist.com intended for the website to make illegal firearm sales easier. This is an attempt to distinguish this case from the litany of cases dismissing suits against website operators who failed to screen unlawful content. As the First Circuit has recognized, however, the allegation of intent is "a distinction without a difference" and does not affect CDA immunity….

The one outlier § 230 precedent is J.S. v. Village Voice Media Holdings, LLC (Wash. 2015), but the Wisconsin Supreme Court rejected it:

In J.S. , which involved claims against the operator of backpage.com on substantially the same facts as in Jane Doe No. 1 v. Backpage.com, LLC (1st Cir. 2016) , the plaintiffs made the same argument as the Jane Doe No. 1 plaintiffs, asserting that backpage.com was deliberately designed to facilitate sex trafficking. The Washington Supreme Court concluded that the plaintiffs' allegation of intent was enough to escape the reach of the CDA…. [But] the Washington Supreme Court ignored the text of the CDA, and the overwhelming majority of cases interpreting it, by inserting an intent exception into § 230(c)(1)….

Sounds right to me. (Disclosure: My students and I filed an amicus brief on the losing side in J.S.)