Condition here is mostly paper condition, with one extra question nothing else on the shelf has: how much of the set is actually there.
The states
Sealed — shrink-wrap intact, never opened. Complete set — every volume present, and you say which numbers. Partial set — say exactly which are missing. Reading condition — read, worn, honest about it.
Completeness is part of condition in this category. A description that does not give volume numbers has left out the main fact.
Paper condition
Spines — creasing, cracking, whether the book has been read flat. The first place wear shows. Page edges — yellowing, foxing, damp staining, dog-ears. Covers — bends, scuffs, curling, price stickers and the marks left by removing them. Dust jackets, where the edition had them — graded separately, like any book. Inside — writing, names, library marks.
Edition is not condition
Standard, deluxe, omnibus, collector's — that is which item you have, not what state it is in. Say both, in that order, and show the publication page so nobody has to take your word for the first part.
Figures and merchandise
Box and figure graded separately. Paint, joints, yellowing from sunlight, and whether it has been displayed. The toys condition guide has the vocabulary.
Questions people ask
Is yellowing a defect?
It is age, and it lowers value. Describe it and show the page edges rather than arguing about the word.
How do I describe a cracked spine?
By where and how far, with a photograph of the book closed and open flat.
Does one missing volume ruin a set?
It changes what the set is. Say the number, price it as partial, and plenty of buyers will still want it.
More: condition words across every category, describing books.