This can't actually be one of the largest stocks of second-hand books. That's just 50 of me, and I only have a couple walls full. He's got 25 rooms, which very much sounds like 50 of me. I'm poor, I'm not special.
I wish I could get rid of them all; when I want to read one again I just pirate a scan. I've literally thought about tightly wrapping them in plastic and burying them in a concrete vault. After the original texts of all "legacy" writing have been deemed too provocative, somebody could discover them and dig them up, show their friends how they've been changed since everything went digital, and shortly afterwards get arrested as a terrorist pornographer.
"This can't actually be one of the largest stocks of second-hand books"
Any library can be described as a second hand store of books and it seems this one is pretty large!
"I wish I could get rid of them all"
You could donate them to the really weird collectors - see OP!
I used to have quite a large collection of CDs (and tapes but they are mostly buggered). It took quite a while to rip them to FLAC but they now take up quite a small amount of disc compared to photos and the rest. A terabyte is not particularly large these days.
"terrorist pornographer" - that is quite a job title.