The decision is Vesely v. Armslist, LLC (N.D. Ill. July 29, 2013) (thanks to the Media Law Resource Center for the pointer). Here’s the Brady Campaign’s theory:

On April 13, 2011, Jitka Vesel, a 36-year-old immigrant from the Czech Republic was shot and killed by Demetry Smirnov, a Russian immigrant residing in Canada who had met Jitka online a few years earlier. Smirnov stalked her to her workplace parking lot where he shot her 11-12 times with a .40-caliber handgun…. The complaint alleges that [Smirnov] illegally purchased from a private seller whom he located through armslist.com, an online gun auction site owned by defendant Armslist, LLC. The complaint alleges that the website’s design facilitates illegal gun sales to unlawful gun buyers with no background checks and no questions asked, and encourages and enables users to evade laws that allow private sellers to sell firearms only to residents of their own state by enticing prospective buyers to search for and find gun sellers throughout all 50 states….

I argued when the lawsuit was filed that it was preempted by 47 U.S.C. § 230, which generally bars lawsuits against Web sites for material posted by their users. I still think that’s so, but the court didn’t reach that theory, because it concluded that the lawsuit was unfounded under Illinois tort law. A key excerpt (paragraph break added):

To determine whether a duty exists, in light of public policy, the Court considers four factors: “(1) the reasonable foreseeability of the injury; (2) the likelihood of the injury; (3) the magnitude of the burden of guarding against the injury; and (4) the consequences of placing that burden on the defendant.” City of Chi. v. Beretta U.S.A. Corp., 821 N.E. 2d 1099, 1125 (Ill. 2004) (citing Bajwa v. Metro. Life Ins. Co., 804 N.E.2d 519, 529 (Ill. 2004)). Here, Plaintiff argues that it was foreseeable that Defendant’s website would enable and facilitate illegal handgun sales — by automatically publishing advertisements posted by private sellers at the national level and facilitating the search of those sellers by private buyers — and that those sales would lead to criminal shootings. This argument is speculative at best. The Court does not accept as true Plaintiffs mere conclusion that Defendant encourages its users to circumvent existing gun laws by enabling prospective purchasers to state. As Defendant points out, “[a] person not licensed under the GCA and not prohibited from acquiring firearms may purchase a firearm from an out-of-State source and obtain the firearm if an arrangement is made with a licensed dealer in the purchaser’s State of residence for the purchaser to obtain the firearm from the dealer.” United States Department of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives, Firearms — Frequently Asked Questions — Unlicensed Persons, http://www.atf.gov/content/firearms-frequently-asked-questions-unlicensed-persons#gca-unlicensed-transfer (citing 18 U.S.C. § 922(a)(3), (b)(3)) (last visited July 26, 2013)…. Furthermore, the magnitude of the burden that Plaintiff seeks to impose on Defendant of guarding against illegal gun sales, by altering its business practice to become involved in sales transactions between third-parties, is immense and, as Defendant suggests, would effectively put it out of business. Accordingly, the Court finds that Defendant owes no duty to the general public to operate its website to control private individual users’ sale of handguns.

I’m not sure about this analysis as a matter of modern American tort law, but it might well be consistent with Illinois law, and in any event I think this would be the right result under 47 U.S.C. § 230.