If you impose liability on the seller, that's way beyond anything our legal system does. Can you imagine suing Chrysler for a car accident caused by a bad driver? The effect of that law would be pretty close to getting knocked down on Second Amendment issues dealing with potential purchasers because of the chilling effect on selling firearms, and then you'd be dealing with all the challenges from the sellers, who shouldn't be held liable for failing to diagnose a psychological condition because, you know, they're salespeople, not psychologists.