Ebay tries to prevent you from selling illegal stuff though. Silk Road didn't. The reputation system was to prevent scams and bad quality products, not to prevent illegal transactions, right?
A large minority of the population (and in some cases, like weed, an overt majority) of the population don't think those transactions should be illegal. "The law is wrong" is sort of the whole point, and why Ulbricht is a quasi-folk hero.