For books, BSR is the standard velocity check. A title ranked under 100,000 in Books typically moves within weeks; something ranked in the millions may sit for a year or longer even priced competitively.
BSR is a snapshot, not an average. A single sale can pull a dormant title's rank from 3,000,000 to 200,000 for a day. Experienced scouts look at rank history charts in their scouting app rather than the instantaneous number.
At a library sale the practical use is triage. With thousands of books and limited time, BSR plus expected sale price tells you whether a book is worth the shelf space, the storage time, and the eventual shipping cost.