No service grades manga, and no numbered ladder is in general use. What buyers in this category actually assess is simpler and stricter.
Completeness is the first grade
Sealed — shrink-wrap intact. Complete set — every volume present, stated by number. Partial set — exactly which are missing. Reading condition — read, worn, honestly described.
A description without volume numbers has left out the main fact. "Near complete" is not information. Describing manga.
Then paper condition
Spines first — creasing, cracking, and whether a book has been read flat. Spine fading across a run is visible immediately when the spines are photographed in a row. Page edges — yellowing, foxing, damp staining. Covers — bends, scuffs, curling, sticker marks. Dust jackets, where the edition had them, graded separately like any book.
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, with the box usually carrying more of the value. The toys guide has the vocabulary.
Borrowing a ladder
Some sellers describe manga using the book trade's ladder — fine, near fine, very good, good. If you do, say so, and remember that good means worn.
Questions people ask
Is yellowing a defect?
It is age, and it lowers value. Describe it and show the edges.
Does one missing volume ruin a set?
It changes what the set is. Say the number and price it as partial.
Are unofficial printings graded differently?
They are a different thing entirely and must never be sold as official. Counterfeits policy.