That shelf of finished books is money sitting still — and a reader somewhere is searching for exactly one of them. Selling online takes about five minutes per book once you know the routine.
Current textbooks, popular fiction, children's books, and complete series sell best. Damaged or badly outdated books mostly don't — donate those instead. If you have twenty-plus mixed books, list them as a single book lot rather than twenty ads.
Front cover, back cover, spine, and one photo of any damage or writing. A phone near a window is all you need. Honest photos are what turn an enquiry into a sale — and what protect your seller rating afterwards.
Check what the book costs new, then price by condition: around half of new for like-new copies, a third for good ones, less for fair. Search your own title on the site first — if three identical copies are listed, undercut them slightly.
On SecondBook, listing is free: add the title (ISBN lookup fills in the details), condition, price, and photos, and buyers contact you directly. Meet in a public place or use cash on delivery with a courier. Mark the book sold once it's gone so you stop getting calls.