One science fiction story related to credit cards could be about a future where credit cards are implanted in people's bodies. They can be used for all transactions just by a thought or a wave of the hand. It's a world where the line between human and technology is further blurred, and credit cards become an even more integral part of daily life.
It varies. Some comic shops may get discounts based on their purchase volume or long-term partnerships with suppliers. But not all shops have this privilege.
It varies from pawn shop to pawn shop. Some might accept comic books if they're in good condition and popular titles. Others might not be interested at all. You'd have to call around or visit a few to find out for sure.