The large range of products on marketplaces encourages sellers to look for ways to improve their products. They are interested in getting as much honest feedback from customers as possible.
To do this, they are willing to trade their goods for the most valuable resource - your time! After all, you, as a tester, are offered to invest your time in the process of using the product, as well as filling out feedback forms on the results of testing.
Thus, between you and the seller there is an exchange - he gets your feedback, which will help to increase sales, and you get 100% cashback, keeping the product.
This is a way for the seller to raise the rating of the product, and for you to get the product for free.