I just checked online and the 80% lowers are as expensive or more expensive then a completed and registered lower. What kind of idiot would pay just as much or more for a product that isn't finished. His only legal option would be then to bring the 80% lower to licensed firearms manufacturer and pay to have the machining process completed and then the lower registered. Based on this video the shop fees wouldn't be that high given the work necessary, but still seems like a waste of money.
https://www.youtube.com/watch?v=N-FTZTiJ37c
Weird!