I had a bunch of shit written but figured I could sum it up this way:
Call sheriff, ask desk sgt. to run SN through all federal databases. If not reported yet, won't show up. That means it had to be stolen within a week most likely.
Call maker, be it ITT or L3 and ask them to check the SN and see who bought that set initially. If a dealer, GTG, if Army, leave it alone without proper documentation from DRMS (or whoever) that shows they released it out of inventory.
That's about all you can do. As far as function, the 6015's had two batteries I was first issued those in 2000-01. Newer ones have one battery. They could look new but be old as shit and you may not be able to tell how much use they've had.
I got mine through TNVC, hand select ITT NE, built on OMNI7 contract (which is tighter than 8) is warranted, has the build sheet and you can set the min. specs, a receipt and they don't act like used car salesmen.
If they are real cheap, you need to ask yourself why. And if you aren't familiar with NODs, how do you know he didn't swap tubes on you? It might not be gen. 3. Unless you've seen the difference yourself, it's really hard to explain.
Personally, these are one of the items I don't buy used. And if I did buy a mil set, then not sight unseen and not without documentation that I can vet.