Say you buy a product listed as "Cool WebCam HD". You might deal with "xXCoolSellerXx" while I would deal with "GadgetSeller51". By leaving a bad review unrelated to the item, you are drowning the actual bad reviews and hurting quality sellers.
Upon purchasing a product on Amazon you will receive a link to review the seller. That is where you are to put your review about the way the seller dealt with you.