Why not have certified users who have a clear history of buying things on the platform that can provide "trusted" reviews? Certainly there might still be a prob

Why not have certified users who have a clear history of buying things on the platform that can provide "trusted" reviews? Certainly there might still be a problem with sellers spamming people to ask for reviews in money but I think it would up the bar substantially. You can then improve this by only allowing reviews from people with known purchases. You can roll this out with two 5-star rating systems and replace the old one once the new one has enough ratings.