Old books are piled up in the warehouse, or if you buy them and don't read them at all, the paper will turn yellow and old. This is called natural aging. For example, the label "Nine products are naturally old" for selling used books is this kind of book.
Nine products are 90% new. Generally, there is no collection value under six products.
For example, before the Cultural Revolution, you bought a Xinhua Dictionary, but you didn't read it. Now old and natural, it can be worth tens of dollars. If you use it, it will be worthless.