There's also low scale review fraud like my sister bought a pet cam and gave it a 3 star review, and the seller started to offer escalating offers for her to remove or increase the rating from full refund, another camera, refund + free camera, and refund + money for the inconvenience.
It does bug me that scummy vendors know my email address and physical address. What's next? They send goons to my house demanding a good review or my dog gets it?