That's unfortunate. Another case of misplaced blame by the anti-gun movement.
I've noticed in recent years that it has become harder and harder to facilitate face-to-face private party sales of firearms via the internet/classified ads. Person-to-person private party sales have been a part of gun-owning culture since before I was born, and the internet just provided a new way for buyers and sellers to meet each other.
Attacks on these basic means of trade have been coming at us for well over a decade now, probably ever since the politicized "gun show loophole" in the wake of the Columbine shooting. There never was a gun show loophole, and there's no way a reasonable person would blame Armslist for the actions of a deranged killer. But, we're lawyering ourselves to death here in America, so I suppose this really shouldn't come as a surprise to me.