Authenticity cannot be settled on a video call. In this category the usual problem is not a forged book but a real one described as something earlier or rarer than it is — and the pages that settle that are photographable.
What to ask them to show
The title page and the page behind it, both in full. Whatever a book's printing history is, it is stated there, and a photograph of it can be read by somebody who knows the series.
The jacket, front, back, spine and both flaps — including the corners and any price that is printed on it. A jacket is replaceable, which is why it is where descriptions go wrong.
The spine, and the top edge of the closed book.
Any signature in the same frame as the title page, rather than cropped on its own.
For a film: the disc or reel, the case and the inserts, separately.
What a refusal means
These are easy photographs — the book is in their hands and the pages are flat. A seller who describes a book as a first edition but will not photograph the page that would show it has made a claim they are not supporting.
"It is a first, I bought it as one" is a chain of trust, not evidence. Ask what it is based on.
What this page will not do
It will not explain how to read a number line or which printings of what are valuable, and it will not tell you how to judge a signature. Those are specialist skills, bibliography is detailed and unforgiving, and a confident general summary is how a buyer pays a first-edition price for a later printing.
Photograph the pages, then ask somebody who collects that author.
What protects you if you get it wrong
A later printing sold as a first is not as described: returnable for 7 days whatever the store's policy says.
Also ask how it will be packed. Damp and a spine crushed in a loose parcel ruin more books on this site than misdescription does.
Questions people ask
Is an ex-library copy a fake?
No, it is a different and usually cheaper thing, and it must be described as one.
Does a signature need paperwork?
It needs whatever the seller has, photographed. Ask where the signature came from.
What about a facsimile jacket?
It is legitimate to sell one, and it has to be disclosed. Ask outright whether the jacket is original to the book.
More: spotting a counterfeit before you bid, Metzee's position on counterfeits, reporting a seller, questions to ask before bidding on books or films.