Even if we have robust red flag laws, there is always going to be someone who manages to slip through the system. Part of that is inevitable, but part of that is due to the nature of our rights that require due process to be stripped of them. I'm not sure we want to set a precedent where we allow rights to be stripped without due process, because that leads us down a very dark rabbit hole. So we need the judgement of salespeople as a last bastion of defense against those who slip through.
Then again, we may be headed down that rabbit hole already due to the nature of technology. It is already possible to manufacture your own magazine fed semi automatic (and probably fully automatic) gun with a couple weeks and about 500 bucks using 3D printers. And that is including the cost of the printer. None of the parts required are regulated as firearm components in the EU, which is much stricter than anything here. Even the barrel can be created with rifling using electrochemical machining and 3D printed jigs. The only way to really prevent someone from a mass shooting using something like that likely involves significant erosion of rights that even people who oppose the second amendment care deeply about.
FGC-9