They say that customer tend to buy products having more reviews and better ratings. To get more reviews and ratings sellers sometimes go for illegitimate reviews and ratings. There are individuals, companies and groups which do this as a business. Individuals are contacted via social networks or are part of groups where group co-ordinator bring works and distribute among group members. Companies have marketers to bring review work.
Reviews business works in 2 ways. One is getting genuine reviews and other as explained above.
Getting genuine reviews
In this buyer is contacted via message or call and is requested to give review for the purchased item. S/he is not given any gift or cash back etc. Seller (or seller company) or hired individual/company does this task of calling/contacting the buyer.
Other way is that buyer is genuine but s/he is given some gift, cash back etc. to review item as positive.
Getting Fake reviews
This is given by buyers who has purchased the product i.e. verified purchase or by users without making a purchase of product item i.e. non-verified purchase. In case of verified purchase, fake buyer is asked to purchase a specific product. Product get delivers and then a review is done by buyer. Seller returns full amount to buyer through other mode of payment. Buyer generally keeps the product. Sometimes there is some amount of commission also given.
This has twin benefits. Firstly, seller is getting good reviews and rating and secondly, sales rank matrix improves because of a sale.