The testers get the refund for the cost of purchasing the products, in addition to a small fee.
The testers buy the products so that the reviews they post get classified as "Amazon Verified Purchases".
One review firm, AMZTigers of Germany boasts of deploying 3,000 testers in the UK alone.
"We help you get verified reviews from real people. Our more than 60,000 product testers throughout Europe specialise in writing reviews quickly and reliably," according to the website of the company.
Amazon said it was committed to protecting the integrity of reviews and had spent 300 million pound in the past year to protect customers from abuse, fraud and other forms of misconduct.
"Our objective is to catch and remove abusive reviews before a customer ever sees it and in the last month, over 99 per cent of the reviews read by the customers were authentic," an Amazon spokesman was quoted as saying.