There is no universal scale here, but there is a strong convention: grade the item and its packaging separately, every time.
The states
Sealed — unopened, seal intact. Mint in box (MIB) — opened, unplayed, in the original box. Mint on card — for carded figures, with the card graded in its own right. Boxed, incomplete — exactly what is missing. Complete, loose — every piece, no box. Played with — real wear, described.
The box is graded separately
Shelf wear, creases, crushed corners, a torn or cracked window, price stickers, sun fading, writing. A box graded in the same sentence as the toy is a box that has not been graded. Describing toys.
What is assessed on the item
Paint rubs, chips and touch-ups. Joint tightness — loose joints are a real fault for anything articulated. Yellowing from light, which is permanent. Stickers and decals, applied or unapplied. Smell.
Professional grading
Services grade some carded and boxed collectibles. It is slow, it costs, and for most pieces it is not worth the fee. Where a piece is graded, say by whom; where it is not, say ungraded.
The number that is not a grade
"9/10" from a seller with no scale behind it tells a buyer nothing. Describe the specific faults instead — it takes one more sentence and it survives contact with the parcel.
Reproduction parts
Not a grade but a fact that changes what the object is. Any replaced accessory, reproduction sticker sheet or modern stand gets its own line.
Questions people ask
Should I open a sealed item to check it?
No. Sealed is a state with its own value.
Is a crushed box still worth selling?
Yes, described as crushed.
Does yellowing reverse?
No. Keep pieces out of light in the first place. Storing toys.
More: buying toys, what they are worth.