Often it's the author themselves using sock-puppet accounts. One of my claims to semi-fame is that I was one of the first to identify a particularly now-notorio

Often it's the author themselves using sock-puppet accounts. One of my claims to semi-fame is that I was one of the first to identify a particularly now-notorious YA author as a barely-literate review cheat. (I won't name him because he has spent almost twenty years since then trying to retaliate e.g. with fake legal threats and doxxing combined with some libel of his own, and I'm not interested in fighting another round when his frequent searches for his own name lead him here.) There's at least one Hall of Shame I know of for authors caught doing this, and it has many entries. For books and many other things you have to simply ignore five-star reviews because the few that aren't fake are kind of clueless ("product arrived quickly"). Often you need to ignore one-star reviews as well because they're packed by competitors and people who couldn't read the directions. There's far more actual information to be found in the middle/mixed ratings.