A guy walks into your gun store.
He's a neo-Nazi and a member of a white supremacist heavy metal band. He also has a minor criminal record. And he's a former military psyops specialist.
Do you sell him a gun?
Yes! Of course you do! It's perfectly legal! Who could ever guess he might use that gun to shoot people who he perceives to be evildoers -- an act of domestic terrorism? No way. Not a chance.