You're working along a good train of thought on how to combat the issue, but unfortunately that's not a viable solution. Contrary to popular belief among gun control advocates, such as myself, there are no shortage responsible gun owners. Punishing them for selling a weapon is a terrible idea.
That said, so is banning guns, repealing the 2nd Amendment, and passing more laws that don't anything to actually curb the real issue with guns. When I said I thought you were on the right track I meant that the answer is in how people are held responsible, or, more specifically, who is held responsible.
The Founding Fathers were strong gun control advocates, much more than anything we do today. They did come for people's guns, and the people gave them up. But we don't need to do that.
We can leave all guns just as they are, even remove much of the legislation, and allow the 2nd Amendment to stay in place. We can even amend the 2nd Amendment to guarantee that we won't pass any future laws restricting the availability of firearms. Because it's about comprise. That's the only way to solve the problem.
In exchange for all of that, all gun control advocates ask is that we also include a provision to the 2nd Amendment that removes the Constitutional protection of firearms as a product. Meaning that gun and ammunition manufacturers can be held liable in civil court without any Constitutional or federal interference. This is already starting to happen, at least on the State level, but it has yet to be heard by the Court. Removing the Protection of Lawful Commerce in Arms legislation would be part of the solution.
This puts the entire gun owning community in charge of their own fate. If they allow irresponsible gun purchasing they are responsible for the consequences. If they want AR-15s to be sold then they understand that the manufacturer will likely be bankrupt if a school gets shot up using one. Manufacturers will also be far less likely to manufacture and sell those weapons so carelessly.
The goal shouldn't be to win an "us vs them" war, because that's stupid. Most gun owners aren't bad evil people hoping schools get shot up. They are generally people who believe in their souls that someone wants to take away something they believe they should have. In their defense many people do, but in "our" defense there's some really shitty people who get ahold of guns too easily. It's not about winning and losing, it's about comprise and progress.
Before someone claims people would go shoot up a school just to bankrupt a manufacturer, it's not that easy to successfully sue someone. Anyone can file a lawsuit, but in reality there are very few frivolous lawsuits that make it to court.