A used book's price follows how many copies exist, not what the rest of its series cost, so the second volume of a cheap set can cost thirty times the first. Buyers who anchor on the first price feel cheated, even though the seller is only pricing what is rare.
A used bookseller stocks the shop by buying whole collections sight unseen, betting the lot holds more treasure than trash. Each trip also means leaving the shop with untrained or unreliable stand-in staff.
A small-town shop earns most of its money in the brief spells of tourist season, festivals and holidays, and must stretch that income across the long quiet months when customers stay home. Because the till alone rarely covers costs, the owner has to add side schemes to stay afloat.
Libraries stamp, sticker, sleeve, tear out pages and mark books 'DISCARD' to keep them safe and track them. To a later buyer this handling is damage. A book that has passed through a library is usually worth less than a quarter of one that has not.
Customers browse the physical shelves to find a book they want, check the same title online for a lower price, and usually leave without buying. The shop bears the costs of stock and space while the online seller collects the sale.