Huh, that's interesting. I
think the only Ingram MAC-type gun on the list is the MAC-11 (and then only as a pistol).
You can take a look at some useful documentation from
CalGuns or the
Attorny General if you'd like.
If everything is on the up-and-up, your buyer might be active-duty military, stationed in California, a prop company, or just one of the very lucky few with a collector's license.
Stay safe!