Most ARs are manufactured to "Mil Spec", so any upper should fit. Uppers can be bought and sold without an FFL, so you could advertise and sell the upper on sites like Gunbroker.com. It's the lower that is serialized, so selling or buying a lower gets into the FFL category unless it's a private sale. Carry-handle uppers are only popular with the miltary buffs, as mounting scopes on them is difficult. You could buy a stripped flattop upper and transfer the barrel and other parts over to it. ARs are modular guns for the most part, but some companies epoxy the parts for some reason, and even pin the flash hiders, etc. Not sure about Olympic Arms guns.
