a) Highlighting how some "Amazon Choice" products have scored really low when actually tested (due to fake reviews) - https://www.which.co.uk/news/article/amazon-tech-with-fake-r...
b) Buying fake reviews linked to facebook accounts - https://www.which.co.uk/news/article/how-facebook-fuels-amaz...
c) Though in the interests of posting counter argument, allegedly Amazon are making some efforts here - https://www.which.co.uk/news/article/amazon-takes-legal-acti...
Personally I try to avoid using Amazon nowadays and buy direct, but previously I've had a least one occasion where I've been asked to leave a 5* review in exchange for a free accessory and we've seen really top notch ripoffs (headphones) that were practically indistinguishable from the originals (did a side by side comparison).