Be very careful even with a standard FFL if you are combining upper and lower receivers yourself for selling.
Yes its stupid, but this will make you a manufacturer which requires a big ol $2,500 minimum per year fee plus some other paperwork.
What makes it worse is the wording from the ATF is so vague its retarded. It says basically this rule only applies if a "majority of your business and sales" are a result of putting together uppers and lowers.
So if I sell 100 uppers a year and only 25 upper/lower combos a year, am I still a manufacturer? The wording indicates I am not, but who wants to be a test case for those dudes?