I bought the transparencies to the top pics from TTWII from ebay.com. The auction finished in the middle of the night UK time, I set the alarm to get up and bid 'live' for them. Paid about $250 for each transparency.
They've blown up to make fab pics
You mean negatives, I take it? Or slides?
In any event, I can see doing that if it were a very special photo, a great pose, and very rare. Although $250 is a bit rich for my blood for a photograph. That's that thing: if it's legit, it's going to cost an arm and a leg. So, first you have to have the disposable income, and then you have to have a special purpose for it, and then you have to determine if it really IS legitimate.
For instance, the photo I posted does not look like the photo paper used at the time of the photograph. Perhaps. So, then it becomes a very arduous process. To me, the photos on the 'net are incredible. I wish I had taken photos of him in life, myself, but that didn't happen. I wouldn't be selling them, of course.
But I can see wanting the negative for a large poster. IF the money is available for that. (There are some lovely calendars out there, and if I want a poster, I tend to use those - make them bigger with the scanner, and it usually works out well, if they use high-quality paper.)