The story
From un- + haul (“showcase of recently-purchased items”). Although haul is used in general for any type of items, unhaul has come to be used particularly for books.
First known use, by meaning
- from 2013 To get rid of (unwanted books).
- from 2012 The act of getting rid of unwanted books.
Etymology from Wiktionary, CC BY-SA 4.0.