Most old books aren't valuable — but a true first edition of the right title can be worth many times a reading copy. Before you price that shelf find, check these signals.
On the copyright page, most modern publishers print a number line like 10 9 8 7 6 5 4 3 2 1. If the 1 is present, you're usually holding a first printing. If the line starts at 3, it's a third printing — a different thing entirely to collectors.
Some publishers leave 'First Edition' on the page across many printings and only change the number line. The words alone aren't proof; the number line and the publication date together are.
For collectible books, the dust jacket can be more than half the value. A first edition without its jacket, or with heavy wear, is worth a fraction of a clean copy. Photograph the jacket, spine, and copyright page for your listing.
SecondBook listings have a dedicated 'notes about this specific copy' field — that's the place for 'first edition, second printing, unclipped jacket, previous owner's bookplate'. Collectors read it before anything else.