Not only Amazon i think it can be done with any shopping website, i think it is not allowed legally to do so as it affects the way a person take his decision whether to buy a product or not.In past Amazon had also sued a website’s named buyamazonreviews.com, buyazonreviews.com, bayreviews.net and buyreviewsnow.com, for doing so, but here i am talking about a different thing here nothing is automated that would raise any sort of problems for the seller. There are few forums out there (Will not name any of them as i have learnt a lot from there) where people bid for such reviews and that’s working quite good.
I don’t know whether it is legal or illegal to do so but the trick is working find and minting lot of money for sellers. For buyers i would suggest just one thing make sure you keep your eyes open before buying a product only on the basis of a user review.
Seller posts (on forums) about paying for AMAZON review cost depends on product to product and seller point of view, most of them look for reviewers from their targeted area. They provide reviewer a system to buy the product for free or by paying less using a promotion code system,where code is already emailed to that person.Once he posts review money is sent through Paypal. Sometimes in case seller don’t have any promotion code system he paypals you the money upfront that you have to refund after it you can post your review.