The marketplace of ideas depends on the open exchange of ideas, in which popularity is based on the value of the argument itself. This understanding fails to account for the bias that exists in all arenas.  In political bodies, for example, party allegiance might determine how one chooses to vote. The concept is therefore too idealistic to be enacted in any society. We cannot say whether it works for doesn't for this reason: there is no precedent either way. Any argument on the subject is therefore pure conjecture.
The prominence of the marketplace in US legal defence shows this is not the case. At its core, the marketplace of ideas is a question of censorship. That it has successfully been invoked hundreds of times to challenge censorship, is proof that it can and does exist. And, that the existence of bias, is irrelevant to its operation.