MadGeniusClub:

Since Amazon first opened its virtual doors, there have been concerns about reviews. Not just for books but for all the products sold through its site. It is no secret that authors have paid for reviews — and some still do. Or that there have been fake accounts set up to give sock puppet reviews. There have been stories about sellers and manufacturers planting fake reviews as well, all in the hopes of bolstering their product rankings and ratings

And:

Amazon now requires you to purchase a minimum of $50 worth of books or other products before you can leave a review or answer questions about a product. These purchases, and it looks like it is a cumulative amount, must be purchased via credit card or debit card — gift cards won’t count. This means someone can’t set up a fake account, buy themselves a gift card and use it to get around the policy.

This certainly will help combat mass fake reviews, since they will no longer be free. As to paid reviews, don’t see how this will change that practice. Here’s a link to Amazon’s official policy page.