However these are definitely not liquid, if you are going to sell them you maybe have to store them for a long time.
Similarly, I'm looking for the complete Collected Works of Carl Jung, and that's got a hefty price too. Maybe one day. :)
I'm sure both of these examples are sitting in some old man's study and are getting sold for nothing at estate sales, if they aren't just thrown in a dumpster or pulped after being donated to a library that can't get rid of them either. But nobody is indexing estate sales.
Out of curiosity, why this specific publication? Can’t you get everything in that collection from other publications (perhaps not in one volume)?
Any scholarly person who loves old hardback books and spends a few decades collecting ones they personally want or need is going to end up with some worthless books, a large number that sell for $10–50 each, and a few that are worth hundreds each. It’s just inevitable, unless they go out of their way to only collect junk.
If I had to guess I’d put the price of the old man’s collection in the $10k–$30k range. But it’s plausible it could be more, if he collected anything rare.
At one point it was hard to get and were selling for $700 - they are now $50.
Had a friend find a bunch of $100 bills in a used book in Salvation Army in SF...
Books by the Foot will sell you books for about $0.20 per book https://booksbythefoot.com/product/shelf-filler-bulk/