Ask on the listing before the show where you can — public questions stay there for the next person, and sellers answer them with more care than a line in chat.
The questions
"Can you hold the copyright page still?" Printing, publisher and year in one frame. It is the single most useful request in the category. Non-answer: "It says first edition on the cover."
"Is the jacket original, and is the price corner intact?" A price-clipped jacket is a different item, and a facsimile jacket is a different item again.
"How are you grading it?" And remember that in this trade good means worn. Ask what they mean by the word. The ladder.
"Is it ex-library, and are there any marks?" Not a flaw to avoid — a category to know about.
"What do the page edges look like?" Foxing and damp show there first.
For films: "What region is it, and have you played it recently?" A disc your player will not read is useless, and the marking is on the spine.
For signed copies
"What do you know about the signature?" Nobody settles that over a call. "It came with the book" is honest and means unverified. Provenance.
What protection covers
A later printing sold as a first, a jacket that was not mentioned, a disc that will not play: seven days from delivery.
Questions people ask
Can I ask for a specific page?
Yes. Flat, square on, even light.
Is a book club edition a problem?
Only if undeclared. Ask directly — the jacket often looks the same.
What about a box of books?
Ask for the spines in rows, readable.