The Amazon review system allows reviews to be shared even if the item was not purchased directly from Amazon. For example, people who received the item as a gift or purchased it elsewhere can leave a review.
You may have noticed the “Verified Purchase” tag next to some reviews. This tag shows that the reviewer bought that product from an Amazon seller. If a review doesn’t have that tag, it means they bought the product from a different site or retail store. In some cases, they may not have purchased the item at all. If someone abuses the review system to post fraudulent reviews, you can always report them.
So the answer to the question is yes. Anyone can review any product, no matter where they bought it. Of course, their reviews are still subject to the Amazon community guidelines, including having spent at least $50 on Amazon in the last 12 months. There’s also plenty of rules about content, as well as the restrictions we mentioned above.
It’s worth mentioning, though, that Amazon always lists “Verified Purchase” reviews above others, giving them more priority. For products with hundreds of reviews, it’s likely that most customers won’t get around to the unofficial ones.