Same thing's happening with thrift store vinyl--slim pickings (maybe 1/20 the volume), and the records there are mostly by sanitized white gospel groups from the 70's.

Theoretically, when aging boomers like me pass on, their survivors will be motivated to call Goodwill or whoever to haul off the lot, but it may be the case that Internet-savvy heirs have read articles about how vinyl is in more demand or a particular rare record has sold for $$$, and so will hold onto those records or try to sell them to a dealer.

Incidentally, I've almost never run across a valuable book at a big box thrift store (I think Goodwill skims them for their online sales). Tiny no-name thrifts and so-called antique malls tend to have a more interesting selection of books (and vinyl).