A complete lower will have a serial number; a complete AR lower is a "firearm" legally.
If you buy a complete lower and assemble (at that point it's pretty much like putting together a model kit) an AR, you could sell it at a later date without too many worries.
If you actually make your own receiver, and you sell the gun, you get in to the issues of "engaging in the business of manufacturing firearms", and as an unlicensed individual, you could be in big trouble. Once the firearm enters in to commerce, it's a whole different story from "building for personal use".
My advice:
1) You should really only thing about "making" a receiver if you're in to the whole "making it myself" sort of thing. Yeah, you can buy a blank forging for under $30, but it requires a couple of hundred worth of taps and bits, not to mention a drill press and/or milling machine. I would stay clear of the 80% AR receivers for the time being, but that's just me. (see note)
2) If this is at all confusing, consider buying a completed receiver and building on that. You can get a Stag receiver for around $100. Just find a friendly FFL that will handle the transfer for you, select from one of the many companies that sell AR parts sets, and away you go.
You will need some special tools, like a barrel wrench, and you should understand things like "torque". Also get a good manual on the gun, which you need anyway. If you're handy with tools, it's not that hard, and it is a lot of fun to shoot a rifle you put together yourself.
Note on "80%": there really is no such thing legally. Either it is a firearm receiver, or it's not. "80%" is a marketing tool to say "some of the hard machine work has been done already". The problem is, legally, there's no definitive line that says how much machining is too much. The ATF will review a part and issue a letter stating whether or not the item is indeed a "firearms receiver". If someone sold an "80%" AR receiver and the ATF decided it was trivial to drill a couple of holes and have a "firearms receiver", they would (and have) determine that was indeed a receiver.