From Guns.com:

In a 5-1 ruling, the court held the federal Communications Decency Act shields the Oklahoma-based website in the case. This had the effect of clearing Armslist of liability in the Brady suit, which aimed to find blame with the company following a 2012 shooting with a firearm purchased from a listing on the site.
A judge in a previous 2016 ruling by a lower court also cited CDA, saying that since Armslist did not create the ad or directly become involved in the transaction, they cannot be held at fault over such third-party content.