Everything that decides a card's value is visible — if the seller shows it the way it needs to be shown.
Ask for
The back. Centring and staining usually show there first, and a seller who only shows fronts may not know that.
Side light. A card lit straight on hides surface scratches and print lines. Ask for it tilted.
Corners, close. All four.
Stillness. A card moving in frame is a card you have not seen.
For graded cards
Ask for the label and grade readable on camera, and for the slab itself — cracks and scratches in the plastic matter to resale.
Grades run on published scales: the main services grade 1 to 10, one of them in half-point steps with sub-grades for each of the four areas. Ask which service, because the scales are not interchangeable in the market's eyes even where the numbers look alike.
For raw cards
Ask what the seller thinks is wrong with it. A seller who answers "soft bottom-left corner and the centring is off to the right" is someone who has looked; a seller who says "it's mint" about a raw card has told you how carefully they looked.
Be careful with predicted grades. Nobody can promise what a card will grade, and a number spoken aloud in a show has a way of becoming the thing you believed you bought.
What protection covers
A card that arrives with damage that was not described, a different card than the one shown, or a slab that was described as intact and is not, is a not-as-described claim — seven days from delivery.
A card that grades lower than the seller guessed is not, unless they stated it as a fact.
Questions people ask
Is a raw card always riskier?
It carries more unknowns, which is why it is cheaper. That is the trade, and it is a fair one when the seller describes honestly.
Why do the same numbers mean different things?
Because the scales belong to different companies with different standards and different market reputations. Ask which one.
Should I ask for the card measured?
For anything valuable and raw, yes. A trimmed card measures short, and a seller with a good card loses nothing by showing it.