The buying and selling of bare/stripped receivers is quite common, especially AR types. Folks actually enjoy building up their own weapons and there is a certain pride in doing so. The receivers are already marked and are bought and sold as controlled items (firearms). Generally, an individual is under no requirement to "re-mark" the receiver after such build out. However, manufacturers and importers ARE. This is something that is often overlooked or poorly understood.

If you are a manufacturer or importer and you build up a stripped receiver for sale, you need to mark the weapon, at the very least, with your name and location. You can go through the arduous process of seeking a marking variance, if you so desire. Please refer to this document for the specifics of what is going on and how to do it:
https://www.atf.gov/sites/default/fi...n-firearms.pdf