As one collector put it to me: “I have more in common with a Beanie Babies enthusiast than I do with a neo-Nazi.” The man with perhaps the most valuable Nazi memorabilia collection in the United States also has what is likely the most extensive collection of “Back to the Future” memorabilia, including a DeLorean used onscreen in “Back to the Future Part III.” Of course it matters to the collectors a great deal that these objects are historical. It’s a big part of the appeal, that nerding out on World War II; it’s the lens they insist on viewing...