Meh, I'd pass on that guy's MP40 lower for several reasons:
1. The price.
2. I think it's pretty unlikely that the veteran would have only brought back the lower receiver and not an entire gun = bullshit story by seller. He probably bought it from Apex Gun Parts
3. When you find a registered MP40 (either C&R or a "tube gun"), it's 99.9% likely it will be an entire gun, so no point to buying a separate lower.
Just my two cents.
[Edit] I love the description of "Appears to only need upper receiver bolt and barrel". I can just imagine a similar used car advertisement:
"FOR SALE: 1963 Porsche 911 body and chassis. Appears to only need engine, transmission, axles and wheels."
[Edit] If you're looking for a transferable MP40, there are two general types:
1. Curio & Relic original all-German MP40s, brought back from WW2, either with paperwork from the U.S. military authorizing the veteran to bring it back to the States, or brought back without permission by the veteran but later registered during the 1968 MG amnesty; and
2. "Tube guns" - German MP40 parts kits imported after 1968, and assembled (with different levels of skill) by American manufacturers on U.S.-made upper receiver tubes from 1968-1986.
C&R guns are more of a collector-type item, while the tube guns are more for shooters, as a general rule.