Yes you can get credits deductions. There are three ways: 1. receiving dislikes on your ratings. 2. disliking other ratings. 3. posting inappropiate content. (In the future) You can spend credits to unlock ranks and achievements.
Yes it is completely free for users. The Master pays for all website costs, by serving ads we try to reduce costs. There is no pay-to-win (atleast not at this moment ;) ). By clicking on the ads regularly you keep the website up and running!