If the facts in the brief report I saw are accurate, then the gun store did violate CA law by selling the "long gun" to an underage individual. That part, I would consider an open and shut matter, either they did, or they didn't violate CA law.
Now, arguing that their marketing was somehow in violation of law is a much different matter and relies heavily, if not entirely on subjective and emotional judgements.
Unless there is a statement from the shooter that they bought their specific gun because of the advertisements, then its all just guesswork, and wish fulfilment.
That is, however, and unfortunately, sometimes enough to obtain a judgement against the (evil
) "big gun maker"....