You can make damn sure they know they are responsible for selling it to someone who can’t pass a background check. You can also reasonably make is so that the firearms can only be sold to a licensed dealer who is required to run such a background check.
ETA: And to bring this back towards the original topic, it’s no surprise that it’s the most fanatic gun nuts who are the ones doing these “protests” against mild inconvenience and weighing it as more important than people’s lives. That’s the story of the NRA and pro-gun lobby in the USA. Any mild inconvenience in acquiring firearms is considered to be tyranny, even if it represents a huge step forward in protecting people.
Point being, if you’re going to bother talking about antifa, you might want to actually know something about the subject. Most of the folks in antifa are the “reasonable and responsible” people you claim are being given a bad name.
