The fundamental problem of the rating systems is how to prevent one party or another (those who receive and those who write reviews) from abusing the system for

The fundamental problem of the rating systems is how to prevent one party or another (those who receive and those who write reviews) from abusing the system for their own advantage? The two obvious problems are: 1) fake reviews 2) blackmailing ("give me free stuff / discount / do what I say or I'll give you 1 star"). While the first problem is obvious, the latter is especially hard and I have yet to see it solved properly. The only place that comes to my mind that has somewhat mitigated it was early eBay that had ratings for both sellers and clients, so dishonest client would receive low ratings. But it also tended to discriminate new users as there's no way to distinguish between a honest, new user and just another single-use account for a scam. Furthermore, it only works on an established system, as you need to have many ratings to filter out noise, so it's a chicken/egg game ("nobody uses platform, because there's not enough users" / "there's not enough users, because nobody uses the platform"). I don't see how these problems could be solved without a 3rd party arbitration, but to do this you have to move back to square 1 (centralized service).