Old rare books where there are single digit copies should enjoy some sort of patrimonial protection just like museum pieces. You can own them but have the state have the option to buy it if you’re about to significantly deface it or destroy it.
Most rare books are rare because no one cared enough about them. Ie most rare books are rubbish.
1. Why are you here?
2. What is the purpose of this comment?
After that you are diving into forum posts etc to see if you can find anyone who has even mentioned owning a copy or having seen a copy.
I don't know what happens to some works. Supposedly thousands, tens of thousands, or sometimes apparently a million or more copies published and yet not a single copy surfaces for years.
It's not just the big players buying up these archives, either. There are a lot of smaller players, especially in the OCR space, who are buying up huge swathes of works in languages which have much smaller digital footprints, e.g. Arabic.