Be prepared for it to not be a legit gun.
(Like seriously have a contingency plan to deal with that possibility upon receipt, if you commit to purchasing one).
I'd want to see a chain of custody or something. Have expert knowledge to verify its authenticity.
Unscrupulous guys will clone serial numbers for that kind of cheddar. That's not illegal for a homemade Title I firearm (currently).
Once it's NFA registered and becomes a Title II firearm, the original NFA registration form shows its pedigree. They're available (redacted) from ATF via FOIA request, but it takes months to get a copy. The seller should be able to provide its history, or at least the form they received it on.
Think 416 parts kit + MR556 semi parts + really perfect clone receiver from an 80% lower.
I'd have to know it wasn't that. 10k(+) isn't parts gun money.