Authenticity cannot be settled on a video call. For posters the usual problem is a modern reproduction described as an original printing, and several of the things that bear on it are photographable.
What to ask them to show
The whole sheet, flat, square on, filling the frame. Not held up, not at an angle, not cropped.
The back. Reproductions and originals differ in ways a specialist reads from the reverse, and it is the view sellers almost never post.
The folds, close up, and whether it has ever been folded at all.
Any printer's mark, censor marking or small print along an edge, photographed in focus.
The measurements taken on camera, edge to edge, because sheet size is part of a poster's identity and a stated size is not a measurement.
What a refusal means
A poster is flat and large and easy to photograph properly. "The light is bad" is a reason to try again, not a reason to decline.
A seller describing something as an original printing who will not photograph the back or the edge markings has withheld the views that bear on the claim.
What this page will not do
It will not tell you what paper of a given age looks like, how fold patterns should fall, or which markings an original carries. Those are specialist judgements and they differ by studio, era and printer.
Photograph the sheet, the back and the edges, and ask somebody who collects that era before you bid.
What protects you if you get it wrong
A reproduction sold as an original printing is not as described: returnable for 7 days whatever the store's policy says, and reportable.
Ask about delivery before bidding, too. A poster must travel rolled in a tube and never folded, and a seller who intends to fold one is about to destroy the thing you bought.
Questions people ask
Is a reproduction worthless?
No, and plenty of people buy them happily. It must be sold as a reproduction.
Is a folded poster a bad sign?
Not at all — many were issued folded. It is a condition question, and it should be described either way.
Does restoration matter?
Yes, and linen-backing or repair must be disclosed. Ask outright.
More: spotting a counterfeit before you bid, Metzee's position on counterfeits, reporting a seller, questions to ask before bidding on a film poster.