Wikileaks is very centralized and technically has not broken any laws.
The activity that will likely occur on open brazzar will be much more illegal then torrenting. There will be much more evidence of such illegal activity as well as actual physical addresses must be used to deliver goods.
Whether or not wikileaks has broken laws is inconsequential. Do you believe states respect their own laws when it applies to themselves? Wikileaks repeatedly commits one of the most egrigious "sins" towards leaders and states: embarresses them, and this is why the US wants to extradite Julian Assange. While the Wikileaks site is not decentralized at the moment, their organization depends upon many decentralized technologies.
Yes, Police will still have to conduct investigations and users will have to be wary on vendors without reputations as they may be undercover agents just like with the silk road. The difference is there is no DPR and no server to find and shutdown the marketplace.
I want investigators the ability to catch "baddies" like child pedophiles and hired assassins. They can and will do so with good old fashion police work.
What will cost a lot of money in prevention is the peaceful exchange of services and property as they can only inefficiently sometimes catch the buyer and catching the vender is much more difficult. Additionally, there is no more "Asset forfeiture" with the vendors profits. The seizing of cash and assets is what incentivizes a lot of the unethical and harmful laws.