I have posted several reviews, possibly 5 of them.
Basically if something is really great, or really bad, then I typically want to express my opinion. However, if a number of other folks have already said what needs to be said then I don’t write a review. If the reviews are bad unilaterally then I usually have no way to review the product because odds are I haven’t bought it.
It is generally fairly easy to tell which reviewers actually have the product in hand as well as which reviewers just have an axe to grind. In a few cases where a reviewer obviously didn’t understand what they were reviewing (typically a case of a student buying a high-level technical book when what they wanted was a junior-high level text with pictures) and where I have seen a copy of the book, I will post an opposing review.
I do evaluate sellers, however, as folks who get accurately-described items out on time, in good packaging, deserve kudos, while those that delay or simply dump something in a box need to know that their methods are not acceptable.