In the broadest category on the site, the honest answer to "what grade is it" is usually that no grade exists — and that is fine, as long as the description does the work.
Where a real scale exists
Some things inside this category borrow a scale from a neighbouring trade: cards from card grading, paper from the book ladder, coins from the numeric scale. If your piece belongs to one of those, use it and name it.
Where none exists
Use the completeness vocabulary instead:
Sealed · Mint in box · Complete · Displayed · Used or played with
Then the specifics: chips, cracks, crazing, paint rubs, yellowing, warping, fading, loose joints, missing stands and inserts, writing, stickers.
The number that is not a grade
A self-assigned "9/10" has no scale behind it. Two sellers mean different things by it and a buyer imports whatever they hope. Describing collectibles.
Grade the packaging separately
Where a piece was issued boxed, the box often carries most of the value and always gets its own sentence.
Certification is not grading
A certificate says somebody asserts something about the piece. It is worth exactly its issuer's reputation, and it is not a condition grade. Keep the two apart.
When professional grading is worth it
Rarely in this category, and only where a recognised service covers the specific thing you have and the gain beats the fee and the wait.
Questions people ask
Can I use a card grading service for a non-card?
Services cover what they cover. Ask before you post anything to one.
Is "rare" part of a grade?
No. It is a claim about scarcity, and edition size is the fact behind it.
What if I cannot identify the piece?
Describe materials, measurements, marks and condition, and say you cannot identify it.
More: buying collectibles, what one is worth.