It is older by a few years, but it is not a two-sided marketplace, it's Blizzard's (+ Activision's) DRM.
FWIW, I did not mean that Steam is necessarily the literal first game marketplace, but to my (limited) knowledge it was the first one that got large enough to become the "default" option.
To be fair Steam wasn't either, in the beginning, which also ruins my argument wrt the iTMS quite a bit: even the Orange Box was still a retail product to most (it's even in the name!) that just happened to be consumed through Steam. Not much different from how iPod users had to use the client app for CDs bought in retail (or far more likely: mp3 from the Napster substitute du jour)
FWIW, I did not mean that Steam is necessarily the literal first game marketplace, but to my (limited) knowledge it was the first one that got large enough to become the "default" option.