Yes, it is too simple. If a vendor wants to pose as a buyer and post fakes reviews if there is no cost to the transaction itself then there is no cost for the vendor to post a fake review.
In Silk Road the cost of producing fake reviews was the commission cost of the sale.
Every buyer who buys something from a seller can review them. It's part of the cost of the goods.
Escrow - same deal.
Edits - this does seem too simple. Is it just naive? Perhaps it doesn't exclude circular transactions and gaming it.