The four things a grader looks at are the four things a seller should show, and most listings show one of them.
Putting a hoped-for grade in the title
A number read as a fact is the number the buyer believes they bought. If you say one, say plainly that it is an opinion.
Comparing raw sales against graded ones
Separate markets with separate buyers. Matching on the player's name and not the parallel or the grade produces a price that never arrives. What cards are worth.
Not naming the parallel
The same photograph exists in many versions at very different scarcities. The parallel is frequently the whole value.
Showing the front only
Centring and staining appear on the back first. So does a seller who has not looked.
Not measuring a valuable raw card
Modern cards are a standard size, so a trimmed card measures short. Measuring costs nothing when the card is fine.
Posting a slab in bubble alone
A slab resists bending and cracks at the corners. Bubble, then a box with the voids filled.
Hiding a card in a bulk lot
Somebody notices. They always notice.
The one that costs most
Flat, moving photographs. A card waved past the lens has not been shown, and an unshown card gets unshown prices.
Questions people ask
Is resubmitting a graded card worth it?
It is a gamble with a fee attached and no guarantee of improvement.
What is a sub-grade?
A separate score for each of the four areas, published by some services.
Do bulk lots sell?
Yes, and they fill a room between the cards people came for.