There is no single numbered ladder in everyday use here. What there is, is a completeness vocabulary — and it does more work than a number would.
The states
Sealed — factory seal intact, never opened. Complete in box (CIB) — case, manual, inserts, disc or cartridge, all present. Boxed, incomplete — say exactly what is missing. Loose — the disc or cartridge alone.
Put the state in the title. It is the first thing a buyer filters on and it moves the price more than condition does.
Then each part, separately
Disc or cartridge — surface marks, whether they can be felt, whether it plays. Case — cracks at the hinge and corners, yellowing, a replaced case, which is always declared. Manual — creases, writing, water marks, missing pages. Label — scuffing, peeling, a replacement.
Testing language is part of the grade
Tested, working. Tested, with faults, and say what. Untested — you did not power it on. Never write "working" for a device you have not switched on. Common mistakes.
Professional grading exists, mostly for sealed items
Services grade sealed and complete games, and it is slow and costly. For most games it is not worth it. If a game is graded, say by whom and what the grade is; if it is not, say ungraded and leave it there.
And a seal is a description rather than a certificate. Where a sealed item is valuable, what matters is what the seller can say about where it came from.
Questions people ask
What counts as complete?
Everything that was in the box when new. If unsure, list what you have rather than claiming the word.
Is a replacement case a problem?
Only if it is undeclared. Write "replacement case" and it is a known fact rather than a surprise in the parcel.
Does yellowed plastic lower the grade?
It lowers the price. Show it in neutral light.
More: describing games, what they are worth.