Here is a copy and paste I got from an 80% website. As with any ATF laws there are gray areas.
Can I sell or transfer my completed lower?
Yes, you can transfer or sell the lower you built as an individual.
But you have to ensure your actions arenít being interpreted as "manufacturing with the intent to sell".
The ATF says that you cannot make an 80 percent lower into a firearm with the intent of selling it. Thatís the key: Intent. And to make things more confusing, the ATF and Federal law have plenty of legalese, statutes, and wordy sections about how "manufactured firearms must be serialized" and "only an FFL can make and sell firearms".
But these regulations only apply to manufacturers, those in the business of building guns. Youíre an individual building your gun for personal use. So, yes, you can sell your completed 80 percent lower to someone later if you didnít originally intend to sell it.
If you bought 10 lowers, machined them all, and posted them for sale online, this would likely be interpreted as manufacturing with intent to sell. Use common sense here.