While some vendors might accept credit cards, many do not. There are ATMs located at McCormick Place; however, to avoid an extra line at C2E2 I suggest you stop at the ATM before arriving at the show. Cash is usually the only form of payment accepted at the autographing area.
Currently the do still have passes left although it will be at a higher charge do to opening weekend. Unsure at this time if the 3 day passes are still available but the single day passes are in ample supply
I believe you can buy tickets once you go through security, though I'm unsure if they have any left. They should take card if they do
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