You know, sometimes Ecosia comes through. It's terse definition of a marketplace as, "the arena of commercial dealings" is that perfect blend of rhetorical flourish, and imprecise language that makes exploration possible.
In our modern world, marketplaces with currencies that last are almost exclusively nations... or are they?
Perhaps a recounting of some currencies that existed in the world long before crypto:
* How about the rupees of Hyrule * Maybe you spent gil in Midgar * Spent some septims in Skyrim * Bought stuff with bottlecaps * and on and on...
If those are unfamiliar to you, those are all games--an art form for which currencies are a core concept.
In the beginning these currencies started off self-contained, but as gaming evolved to multi-player games, the currencies in the games, and the items they could purchase became transactable within, and outside of the game itself.
If this sounds weird it probably is, but I, and the guy I knew who paid his rent with the money he earned fishing in World of Warcraft for half of the aughts, assure you, in-game economies are real.
So now the question is how is a game a marketplace, and to answer that we need to answer What Is A (Video) Game?