I like this system a lot.

I always considered I would do something similar if I owned a used book store. Each year would usher in a new colors. All books acquired that year get that colored dot on the inside page.

Some 5 years (or so) on I could easily go through each shelf of books and find the ones that were not moving. These get one last chance (a year?) in a bargain bin before then they go to Goodwill or wherever.

Otherwise a used bookstore can remain in a "picked over" and cluttered state.