While there are absolutely benefit of a system like this for legal sales, the problem is most sellers of legal goods have many existing outlets that they can use. While they might not like the fees, most accept them as the cost of doing business. Since there are few organized marketplaces online for illegal goods, the first adopters (as with Silk Road) will most likely be the black market, which will likely stigmatize the marketplace, making it far less likely to be adopted by legal sellers.