As far as I can see, the essential problem with decentralized anonymous marketplaces is that reviews need to cost money. Plain and simple. Good reviews have great value, so they need to have a non-zero price. A vendor's review history is the indicator by which you assess whether a vendor is trustworthy or not, and a vendor should have to pay to achieve a trustworthy reputation - if good reviews are free, and you can…
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.