A so-called 80-percent receiver is an AR-15 lower receiver that is 80-percent finished. Depending on the material and the manufacturer, you can finish them yourself using tools ranging from a mill to a Dremel tool. In addition to the satisfaction of “doing it yourself,” an 80 percent lower can be shipped straight to your door with no FFL involved because it’s not a “firearm” until the machining is finished. Because it’s not a firearm, there is no serial number.
According to ATF, “Firearms may be lawfully made by persons who do not hold a manufacturer’s license under the GCA provided they are not for sale or distribution and the maker is not prohibited from receiving or possessing firearms.” That means you can finish these lowers for your own use. You can’t finish one and give it to someone, and your machinist brother-in-law can’t finish your receiver for you.