An 80% lower receiver refers to an item that some may believe it has an incomplete stage of manufacture that meets the definition of firearm frame or receiver in the Gun Control Act of 1968.
FFL requirement to build a firearm for personal use?
The answer is No. Building the firearm for yourself and your own personal use is legal under the Gun Control Act of 1968. This is on the ATF’s website under their FAQ page. You can see it here: http://www.atf.gov/firearms/faq/firearms-technology.html
There is no requirement to transfer through a Federal Firearms License Dealer with all the federal paperwork unlike its counterpart, the complete lower receiver.
As long as the following laws are met there are no problems:
You can legally own the firearm in question in the state in which you live.
You build the firearm for your personal use and for yourself only.
Mark your firearm somehow so that should it ever be lost or stolen, you have a way to identify it. Importantly, be sure to report its loss to your local law enforcement. Please check your local and state laws.