There is a factor that is not being discussed at all. All of my Sandman books are either OO or picked up by me between 2000-2005 or so when all 80s and 90s books were being sold at fractions of their highs.
I do not have an 8(b) because they were never available cheaply, and never made it into bargain bins. I have only held two in person - both at Andy Ash's Sheraton show in the early 90s and both were $100. This was a time when I had never paid more than $20 for a book.
With them being rare as hen's teeth when they were relatively new, I don't see how any data, whether it is completed sales on auction sites or anything else, is more than pure speculation. And I certainly don't see why we should assume a higher than normal attrition rate for something that was HIGHLY coveted from the moment it was known to exist.
If it were me, I would be trolling the former goth girl soccer moms for copies.