Ask on the listing before the show where you can. Public questions are read by everyone who arrives later, so sellers answer them carefully.
The questions
"Can you show the indicia?" The printing is printed inside the book. A cover tells you nothing about which printing you are bidding on. Non-answer: "It's a first." Good answer: the page, held still.
"Can you show the spine, lit from the side?" Spine ticks and stress marks are where condition lives in this trade, and they do not appear under flat light.
"Has it been pressed, cleaned or restored?" All three are normal and all three must be declared. An undeclared restoration is the thing that ends a sale in this category.
"How would you grade it, and do you grade?" A band from somebody honest is worth more than a number from somebody hopeful.
"What are the page edges like?" Colour and foxing. In an Indian climate this matters more than people expect.
For slabbed books
"Which service, what grade, and is the case intact?" A cracked slab is damage even when the book inside is fine.
What protection covers
A reprint sold as a first, undisclosed restoration, damage not shown: seven days from delivery.
Questions people ask
Is a reading copy worth bidding on?
Yes, at reading-copy prices. Low grades of sought-after issues have a real market.
Can I ask for a specific page?
Yes — held flat, square on, in even light.
What if a seller will not take it out of the bag?
For anything valuable, that should lower your bid.
More: buying comics, describing condition.