What you can't buy is the receiver. The receiver is the essential part of the firearm. It's sometimes also called the action. You could purchase the barrel, trigger or components, etc. The real problem is not about buying those other parts, because they are useless without a receiver. The issue is a partially finished receiver that, up to a certain point, is simply a block of aluminum before it becomes a firearm.
That issue needs to be addressed to ensure that only licensed, background-checked and pre-screened individuals have the ability to possess not only completed firearms, but also potential firearms in whatever form.