TL;DR — A search bar that teaches users what to search for, before they type a single character.
As an used bookstore shopper I want to understand what the search can do, so that I can find books I didn't know I could look for.
Why rotating placeholders?
Most users assume search means title. But a second-hand catalogue has real depth: authors, genres, ISBNs, moods, price ranges. Rotating placeholder text cycles through a different entry point on each load, silently expanding what feels possible without a tutorial, tooltip, or onboarding screen. Threads uses the same mechanic to surface trending topics directly in the search field.
Why these six hints?
Each covers a different mental model: author (most natural), genre (browsing without a title), ISBN (power user with a phone), price (the second-hand value prop), mood (how people actually feel about books), award (unexpected, but it sticks).