The Wisconsin Supreme Court rejects a lawsuit against Armslist, and upholds the First Amendment in the process:
Daniel v. Armslist; Wisconsin Supreme Court Case No. 2017AP344
"¶51 Daniel's negligence claim is simply another way of claiming that Armslist is liable for publishing third-party firearm advertisements and for failing to properly screen who may access this content. The complaint alleges that Armslist breached its duty of care by designing a website that could be used to facilitate illegal sales, failing to provide proper legal guidance to users, and failing to adequately screen unlawful content. Restated, it alleges that Armslist provided an online forum for third-party content and failed to adequatelymonitor that content. The duty Armslist is alleged to have violated derives from its role as a publisher of firearm advertisements. This is precisely the type of claim that is prohibited by § 230(c)(1) [of the Communications Decency Act], no matter how artfully pled.....
Regardless of Armslist's knowledge or intent, the relevant question is whether Daniel's claim necessarily requires Armslist to be treated as the publisher or speaker of third-party content. Because it does, the negligence claim must be dismissed."
In short, one cannot blame a website for the criminal acts of third parties simply because they used the site.