You don't have to buy each product for review,that probably wouldn't be a good idea indeed.
But there are many things that you can understand and make people understand with simple language consider a good review,I assume before you review anything you must make sure top ten things about your upcoming review.
-Be Honest
-Gather All The Information|Use Your Wisdom
-See The Other Video Reviews
-Be Informative & Detailed
-Don't Judge Other Reviews
-Do Not Copy Online Content Make Yourself
-2x Check Your Opinion About Service/Product
-Make Sure You Belong To The Niche
-Use Creative & Meaningful Title
-Present Your Verdict.
