Yeah, and our discussion enters the area of question "do we need the law and law enforcement - police, courts etc. to prevent activities that harm other people?"
If such a marketplace will be decentralized/distributed AND anonymous, I find it very difficult even to moderate - how can you stop banned people from creating new, fake identities and continuing their plan?
Would a voting/rating/trust system be enough?
Other ideas?
IMO I think realistically it would need a group of mods. Anything that blatantly breaks the rules should be removed immediately, and items that are in a grey area should be voted against.