Locke - you are mistaken. First, they are 80% finished receivers, not 90%.
Secondly, the rule states that if you create (at home, by yourself) your own finished receivers for whatever the weapon is, you CANNOT sell them. To do this, you need to legally become a firearm manufacturer through the ATF. This IS NOT an easy process.
If you, as a private citizen, build one of these receivers, you CANNOT sell it, you CANNOT even will it to your spouse, children, etc. If you ever decide that you no longer want said receiver, you are legally bound to destroy it. Go against these rules and you violate law and the ATF will put you in prison.