A lot of older books that are now out of print often run many hundreds of dollars, if not more. For example, I've been trying to find a complete unabridged edition of Fraser's Golden Bough, which isn't
that niche - you'll find it cited somewhere in any work on mythology- and it seems to run in the high-hundreds to low thousands. A quick look shows a first edition selling for 12k all by itself.
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.