What's the scam here?

[theGinsue]

Odd
I would tell him:
It is not offered for sale/shipped piecemeal.
It is purchased and shipped to your receiving FFL as a complete firearm or no sale.

That's how I'd go about it.

He get the upper, refuses the lower, calls the credit card company and claims he never got either, fighting the charge.

He may not be able to own a lower or just wants the upper.

I considered that this is likely something about wanting the upper and refusing the lower (just not going in to buy it as this is a prohibited person or something.
It also may have something to do with where he lives and a requirement to get a "print" of the rifling and/or shell casing marks (does Maryland still do that?) or something similar. By getting the upper directly from you there is no way to trace the rifling or the marks on the casing without first obtaining the upper.

Buyer wants his FFL bud to 4473 it as a pistol or "other" and that can't be done when it arrives a5 the FFL with your rifle upper attached.

Hmm, that does make sense and I hadn't considered that as a possibility.