At library book sales, ex-lib copies sit alongside donated copies and are not always separated. Telltale signs include a taped-down mylar dust jacket, a Dewey label on the spine, a 'WITHDRAWN' or 'DISCARD' stamp on the edges or title page, and adhesive residue inside the covers.
For reading, ex-lib copies are usually excellent value: libraries buy reinforced bindings that survive dozens of circulations. For resale, the picture is different — most marketplaces require ex-library status to be disclosed in the listing, and buyers discount it heavily.
Practical inspection: check the text block edges for stamps, open the front and rear boards for pocket residue, and confirm no pages were torn when a security strip was removed. A clean, unmarked interior with only a spine label is the best case.