Three requests get you most of the way, and one question matters more than people realise.
Ask for moving light
Not a brighter photograph — a light moving across the surface while the coin is held still. Lustre, hairlines and cleaning all reveal themselves in the movement and in nothing else.
Ask for the edge
Reeding, lettering, or plain, and its condition. It is the least photographed part of a coin and it carries information about both the type and whether anything has been done to it.
Ask whether it has been cleaned
Directly. A cleaned coin is worth substantially less than an original one, and the answer tells you both about the coin and about the seller.
"I do not know, it came to me like this" is a fine answer. Evasion is also an answer.
What drives value
Rarity within the series, not age. A common 200-year-old coin is worth less than a scarce recent one.
Mint mark and year together — one without the other tells you little.
Grade, where assessed. The long-established scale for coins runs 1 to 70.
On fakes
Cast and struck counterfeits both exist, and weight and diameter against the published standard are the checks that matter — which somebody has to physically perform. That is not something to settle from a blog post or a video call.
For anything where authenticity carries the value, buy graded or buy from somebody who will stand behind it, and rely on the protection for "not as described".
Questions people ask
What does uncirculated mean?
Never used as money. Not the same as shiny, and a cleaned coin is not uncirculated.
Should I clean a coin I buy?
No, never. Cleaning removes the original surface permanently and typically costs a large share of the value. A dull original coin is worth more than a bright cleaned one.
Is toning good or bad?
Natural toning is usually desirable. Artificially induced toning is not, and distinguishing them takes experience.
What if it arrives and it is not what was described?
Seven days from delivery to raise it.
More: provenance.