Interesting topic, and one I have had to wrestle with. There's different ways to look at this, and every car owner has different priorities & goals. If someone called me and said they had something that belonged to my specific '65 and they wanted to sell it to me for $6K I might think they believe they have me over a barrel and are attempting to gouge me. Looked at from another perspective, I could say that person only has 1 prospective customer so their chances of getting top dollar are pretty slim if I am not interested. I can see both sides and it really depends on how far the owner is willing to go to get the car back to "original".
In this case, the car will run just fine without the manual. With rising values these cars have become investments, and the expected ROI has to be there to spend $6K on a document.
Good observations. I recently negotiated on a part for my '65. The seller reminded me several times how rare and unique the part was. I pointed out that buyers are equally rare for his part. A large percentage of '65's are restored or at least in a condition satisfactory to their owners. How large is the market for very specific items like a steering wheel? I think my point of view prevailed as I was able to secure the item and a "reasonable" number. Your point is well taken, the car runs just fine without the manual.
There are a lot of enthusiasts who are not as passionate about history or documentation compared to the small sample of opinions here. On the other hand the individuals who own '65's are likely passionate about the history of their particular car. Not to say there aren't rich guys who simply want to own a '65...
Hey, I happen to need a manual. If the the "proper" owner isn't interested, so be it!