There's a trade off in moving from centralized to distributed markets. If you look at the Hidden Wiki right now, you'll see that it's spammed by a "vendor" named Eris. You can follow the links to his hidden service. Nobody knows anything about this vendor. Would you send BTC to him? The big value add of SR is that it's a central repository of ratings and reviews that builds confidence in making transactions. If you run your own private market, who should believe your reviews? Will there even be any? Will the whole thing depend on an independent review system? The social and behavioral economic aspects of anonymous markets are another hard problem, but this one can't be solved by math or coding skills.