Cards have the most standardised condition language of anything sold here, which makes the honest description unusually easy — and the dishonest one unusually visible.
What is assessed
Centring — how evenly the borders frame the image, front and back, often given as a ratio. Corners — sharp, soft, rounded, or folded. Edges — chipping, which shows white and shows worst on dark borders. Surface — print lines, scratches, dimples, creases, gloss and staining.
The scales
The main services run 1 to 10, with 10 at the top. One of them grades in half-point steps and publishes a sub-grade for each of the four areas, so a card can carry both an overall number and the four behind it.
Numbers are not interchangeable between services. A grade means "this service said this", and the market prices different services differently — so always say which service alongside the number.
What a grade is not
It is not a guarantee about anything except condition at the time of grading. It is not an authentication of value, and it does not transfer to a different card that looks similar.
Grading a raw card yourself
You cannot. What you can do is describe:
Sharp corners, centred slightly left, one small edge chip at the bottom, clean surface under side light.
That sentence is more useful than any guessed number and it cannot come back at you. A number in a title is read as a fact. Common mistakes.
When grading is worth the cost
When the likely gain clearly beats the fee and the wait, which for most cards it does not. Selling raw with an honest description is a perfectly good answer and this room understands it.
Questions people ask
Can a card be re-graded?
Services allow resubmission, the outcome is not guaranteed to improve, and the fee is paid either way.
Does a slab protect the grade?
It records that service's judgement at that time. A cracked slab is damage even when the card is fine.
Is a 9 twice as good as a 4.5?
No. The scale is not linear in price or in meaning.
More: describing cards, what they are worth.