How do legitimate people get their first positive review?
Police could set up a ring of their own accounts, facilitate fake transactions amongst each other, leave positive reviews, take down one large player, then shut that ring down. Imagine hundreds of these rings within the network.
If they're also relying on WoT-like reputation system where peers rate peers, and reputation's calculated from those chains of trust, massive sockpuppet attackers won't be able to quickly erode trust, as they'd generate that trust only throughout their own circle, having minor impact on others. They'll have to play by the rules for a long time to spread connections through the network.