Potentially, if they deliberately sold to criminals with knowledge (or a situation where they should have known) that the weapons would be used for criminal activity (as a primary purpose).
However, this case seems a bit complex (though the article is pretty biased in making its case, but I'll take it at face value). The guy was dumb (my opinion) to sell it on the forum he did, but he did (apparently) make efforts to prevent its use for criminal purposes. But selling on a forum where one of the primary discussion channels is about criminal hacking, it should've been obvious that was a bad place to advertise this tool.
Not really disagreeing, but if I start a gun shop in a bad neighborhood (and then naturally a higher % of my customers will be more criminal in nature) does that make me liable?
Potentially, if you have knowledge of their intended use for the weapons. You likely won't know, or can put yourself in a position where you can plead ignorance (meet at least the legal standards for sales with background checks and such).
You have no obligation to sell in the first place. If a man comes in talking about their intent to shoot up a place, and then comes to you at the counter and asks to buy a gun, you can always turn him away. That's not likely to happen, but that's essentially what happened on the forums. People discussed how to (with malicious intent) hack other computers, and then purchased this guy's software (or pirated it). Now, again per the article, he tried to prevent that use (which is why I say in this case he seems to be just dumb), but it's still a highly questionable group of people to sell and advertise to.
However, this case seems a bit complex (though the article is pretty biased in making its case, but I'll take it at face value). The guy was dumb (my opinion) to sell it on the forum he did, but he did (apparently) make efforts to prevent its use for criminal purposes. But selling on a forum where one of the primary discussion channels is about criminal hacking, it should've been obvious that was a bad place to advertise this tool.