Booksellers have used roughly the same ladder for decades, and one rung of it traps every newcomer.
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 of them named. Good — complete and readable, with real wear. This is a worn book. It is not "good" in the everyday sense, and that mismatch causes more arguments in this trade than anything else. Fair / poor — heavily worn, or a reading copy only.
Because good means worn, write the faults out rather than leaning on the word.
The jacket is 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 only one grade hides which.
If there is no jacket, say no jacket. Not "jacket not pictured".
Words to use precisely
Foxing — brown speckling, damp and age. Cocked — the spine leans; the book will not sit square. Bumped — a corner flattened by impact. Shaken — the text block moves in the binding. Ex-library — library marks and fittings. Price-clipped — the jacket's price corner cut off.
Films
Grade the case and the disc separately, the same way. Sealed, complete in case, disc only. For the disc itself, say whether you have played it recently, and say if the surface has marks.
Say the flaw out loud
A flaw named on camera costs a little. A flaw found in the parcel costs the sale, the rating, and sometimes a refund. Why describing condition honestly works.
Questions people ask
Can I just say "used, good condition"?
You can, and it is the phrase most likely to be read two different ways. Name the faults instead.
Does a remainder mark lower the grade?
It is not damage, but it is a fact about the copy. State it and let the buyer decide.
How do I grade something I am unsure about?
Give a band and say you are unsure. "Very good to good, I am not a specialist" is honest and defensible.
More: condition words across every category, buying books and films.