This. This more than anything. Amazon is now in a position not to need reviews from people that currently own that book. Or better still, there is a point where there is a verified purchases threshold that triggers the hiding of unverified votes.
Yes it still means a person can buy the book, then give it a bad review, then return it, however the effort to do this would be higher AND there would be a purchase pattern that begins to show itself.
You can easily detect that. If a person buys a lot of books and make a lot of 5 star or 1 star reviews, then returns them, that's clearly abuse and you ban them.
Or just wait until they have a lot of books and then reject every return. Their purchase history would tell a compelling story for the credit card bank.
Yes it still means a person can buy the book, then give it a bad review, then return it, however the effort to do this would be higher AND there would be a purchase pattern that begins to show itself.