According to Wikipedia, Amazon.com was incorporated for the first time in 1994 and opened for business in July of 1995. Reviewing became an option apparently a short time later. Here is the oldest review anyone seems to have found on Amazon.com (from October of 1995).
Not particularly fantasy-related, of course, but good enough for a second post on a holiday, especially after posting a review like I did this morning (reviews really require the most effort of any type of post in this blog).
Now I've got to get to working on the manuscript for my second novel. (Gotta do something while waiting to hear back about the submission of the first one.)