No one can stop you from buying reviews, but it’s against App Store rules. Your app may be expelled from the App Store (according to the paragraph 3.10 of App Store Review Guidelines).

Why buy reviews?

• to increase customer rating;
• to increase review activity and break into the top app lists.

If you decide to manipulate reviews, you will be doing it at your own risk. Here you can check out app review service providers and their prices.

We don’t recommend buying reviews for a sophisticated product or an app in a foreign language - comments might be irrelevant. Some providers may post reviews prepared in advance. Confirm all the details with your provider.

Resources:

• Apple Is Taking Action Against Fake Ratings On The App Store
• Can you pay to get positive reviews in the app store?
• Tried buying reviews from fiverr?