In an open and free market most of the stuff sold "is crap", but people generally discover valuable things and will equalize to the right intersection of value and cost, all things equal.
The big problem with a big marketplace is when it obscures the value part of that balance. How do I determine which of these 30 applications has the best value/cost balance?
Isn't straight-up downloads, because who knows how many of those people still use the app. Isn't necessarily rating, because we know those responses don't really answer the big question.
A small curated market solves the hard question (which is most valuable for the price?) but short-cutting the question. The user doesn't need to ask or research, because by the nature of having a small, curated marketplace, the marketplace organizers rely on their own credibility as curators.
Like the news industry, whose job it was to curate the news, eventually a few small curators float to the top and then attract more quality curators, growing that marketplace.
But it's still inherently weak, because you're still not supporting the user answering their own question - which of these has the best value for the cost?