I'm in a situation where there is no accounts. There is only offer-centers (which need quality ratings) and offer-takers. Every time an offer is taken a transaction is taken there is evidence of it, it's possible to have per-transaction reviews.
There is some cost to faking a transaction, but having a good rating is probably worth more.
The question is pretty multi-faceted. The core question is "how do I do reviews well". It breaks down into questions like "How do I deal with fake reviews, positive and negative" and what examplifies trustworthiness?
Links to descriptions of rating systems like eBay, Reddit's, Amazon, etc, are valuable answers too.
(Note: I don't think any single answer can be best, but I hope people will offer perspectives on reviews)