How do credit/debit cards satisfy the proposition of a decentralized marketplace?
They're something that most people are actually using, allowing the marketplace to actually conduct commerce and exist.
And again, how do credit/debit cards—which are centralized systems—satisfy the proposition of a decentralized market?
"And again, how do credit/debit cards—which are centralized systems—satisfy the proposition of a decentralized market?"
They allow the market to actually exist, by using something that the people are actually using.
Clearly not for the people using it right now.
What benefit does this actually give me in my everyday life where I don’t need to buy toilet paper anonymously?
Are you asking why someone would want to transact anonymously?