How does the FDA decide who can buy nic concentrate and who cannot? They would have to license suppliers and companies that mix for resale. That would take time to set up. What would be the criteria for who gets licenses? There are more problems. If they ban shops mixing their own many would go out of business. If they also ban mail order or make it too hard most people would have no place to get vaping products. In the mean time cigarettes would be available 24/7 on every street corner.
They could require a chemist's license to purchase, though through the six degrees of separation I'm betting everyone knows someone who knows someone. For example, I know two. lol