This trade has no single numbered scale in everyday use. What it has is a completeness vocabulary and a habit of grading each part separately, and that is usually more useful anyway.
Completeness first
Sealed — factory seal intact, never opened. Say nothing more than you can back. 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 that word in the title. It is the first thing a buyer filters on.
Then each part
The disc or cartridge — surface marks, whether they can be felt, whether it has been cleaned, whether it plays. The case — cracks, especially at the hinge and corners; yellowing; a replaced case, which should always be declared. The manual — creases, writing, water marks, missing pages. The label on a cartridge — scuffing, peeling, a replacement label.
Testing language
Tested, working — you ran it. Tested, with faults — you ran it and something is wrong. Say what. Untested — you did not run it. Price and describe accordingly.
Never write "working" for something you have not powered on. That is the single sentence that turns into a return.
On grading services
Professional grading exists for sealed and complete games. It is slow, it costs, and for most items 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.
Questions people ask
What counts as complete?
Everything that was in the box when it was new. If you are unsure, list what you have rather than claiming the word.
Is a replacement case a problem?
Only if it is not declared. Say "replacement case" and it is a known fact instead of a surprise.
How should I describe yellowed plastic?
Plainly, with a photograph in neutral light. It is age, it is common, and hiding it does not work.