Better idea: Only verified buyers who have been on Amazon for X amount of time (or some other process to gain trust) should be allowed to leave reviews.
I never experienced it myself, but I’ve seen reviews claiming they had a letter in their package that they’d get X currency if they leave a 5-star-review.
Sorta similar but different, I left a 3 star review for something (cheap headphones that were OK but not great), and got a few emails after it offering me credit etc. if I changed it to a 5 star review.
I didn't, but did report it to Amazon (no idea if they cared at all.)