Books have a condition vocabulary that booksellers have used for decades, and almost no third-party grading at all. The ladder is the whole system.
The ladder
As new / fine — as it left the publisher, no defects. Near fine — very slightly short of that, and you say how. Very good — a used book with small faults, all named. Good — complete and readable, with real wear. This is a worn book. Fair / poor — heavily worn, or a reading copy only.
The trap is permanent and worth repeating: in this trade good means worn, and a buyer reading it in the everyday sense will be disappointed by a book that was described accurately.
Two grades, not one
The book and the dust jacket are graded separately, written as book/jacket — "near fine in a very good jacket". A jacket can sit a grade or two below the book, and giving one number hides which.
If there is no jacket, the words are no jacket, not "jacket not pictured".
The faults that have names
Foxing, cocked, bumped, shaken, ex-library, price-clipped, remainder mark. Each means something specific. The vocabulary.
Because there is no service
Nobody certifies a book grade, which means the description carries all the weight. Name the faults rather than leaning on the word, and photograph the copyright page so the buyer can see what they are grading.
Films
Graded by completeness rather than on a scale: sealed, complete in case, disc only — and the disc described separately for marks and play.
Questions people ask
Can I say "mint" about a book?
The trade's top rung is fine or as new. "Mint" reads as somebody outside the trade.
Does an ex-library copy have a grade?
Yes, with ex-library stated separately — it is a category, not a fault.
Should I grade conservatively?
Always. An under-promised book arrives better than expected.
More: buying books, what one is worth.