The actions of a mass shooter are no more the fault of the manufacturer or seller of ammunition than a car manufacturer is responsible if someone decides to ram his car into a crowd, or a match manufacturer if someone decides to block people in a building and light it on fire.
I think that background checks are marginally effective. While they might keep people who have committed gun crimes from legally obtaining guns, they do nothing to keep the psychotic mentally ill from getting guns. In many cases, these people are giving off signals that they are about to become violent (or commit some other horrific crime, like Edward Snowden or Bradley Manning did), but, thanks to various laws, it is nearly impossible to stop them. The mentally ill cannot be kept confined, because the ACLU successfully sued against the practice as “unlawful imprisonment.” And HIPAA laws prevent mental health information from being included in background checks.
Instead of suing the shop that sold the ammo, how about activating against the laws that make stopping a violently mentally ill person almost impossible to do before the fact?
Maybe they could have sued the movie theater for not having sensors on the exterior exits to alert management that one of them was open to the outside. Makes as much sense.
Not to mention, in some states, they are spreading the net wide as to who is mentally ill or disordered. A vet sought treatment for depression after his wife died, and LE showed up at his home to confiscate his weapons under the pretext that as a mentally ill vet, he might go off on people.
This same government can’t be bothered to check illegal aliens for diseases formerly unknown in this country, and keep them out, or in some school districts, require the lice-ridden children of the illegals to show vaccination records before they can attend (but the children of citizens still have to show them, even though they’re much less likely to have some exotic disease or some super-virulent strain of TB, etc, and can just sit next to your kid and cough Super TB all over them, or give them lice. They showed pictures of some of the illegal kids, the lice were just crawling down their faces, it was that bad, and it honestly didn’t appear to faze them).
Because that's pre-crime law, and it's more dangerous than the problem it's trying to fix.