According to info that I have it is unlawful to sell a lower (or any other firearm) that you have made. You would then be in the business of manufacturing a firearms. This requires you to have a type 07 FFL.
Once you make an 80% receiver its yours forever
As I understand it, you can't make one with the intent to sell it without the type 07 FFL but if you decide to trade/sell it somewhere "down the line" it is allowed. In this case it would probably help to have some kind of identification mark/serial number as per ATF regulations. Always check the ATF regs yourself before taking any action.
JMHO
-Ron