Personally, I'd let the guy keep it if he treasures it that much, unless you simply *must* have an Apple /// to satisfy some deep craving in the darkest recesses of your soul. Save it for someone who already has a ///, already loves them, and needs it for parts.
How well do you know this guy when it comes to the stories he's telling? My guess is that "this III was used in an engineering lab during the most part of the 80s, then placed in storage for 20+ years" translates to: "I bought this off a pallet at the local government surplus sale and it's been sitting in a rented storage unit until I could get around to hosing it off and foisting it off on a sucker. I never *had* any manuals or disks, and a sticker on the back *said* it came from 'a lab'."