According to critics, weaknesses of the eBay feedback system include:
Small and large transactions carry the same weight in the feedback summary. It is therefore easy for a dishonest user to initially build up a deceptive positive rating by buying or selling a number of very low value items, such as e-books, recipes, etc., then subsequently switch to fraud. eBay has since restricted digitally-delivered items to classified listings, which do not involve feedback.
Sellers can also attempt to boost their own feedback by buying their own items with alias accounts, and leaving positive feedback for their main selling account
from Wikepedia article on Ebay criticisms.
It's pretty obvious also that customer feedback has similar problems on Amazon and Trip Advisor: lots of vendors giving themselves glowing reviews. Trip Advisor was recently fined for obvious fraudulent ratings.