Even in real-life, you're never sure, when someone recommends a product, be it a salesman in a brick and mortar store ("this one here is a lot better then that one there", even at the same price, could mean different profits). With platforms like youtube, you have atleast minimal trust in a few youtubers you follow regularly (and this literally means "a few"), but doing a random search means 80% chance the reviewer got sponsoder or atleast received the product for free in exchange for a good review. Star systems on sellers pages have their own sets of issues... usually people don't write reviews at all if the product is good, and complain only after it breaks (unless they're 'paid' for the review).