So far the only buyer/seller rating system that I trust the most is the system implemented at
astromart, but that system comes at a price - some inconvenience and a very elaborate software tracking system.
Here's what I understand:
1. Firstly, members cannot register with free email accounts, so no hotmail, gmail accounts etc are allowed. Only ISP or legitimate company accounts are accepted. That limits the possibility of people registering multiple accounts.
2. When you respond to an ad through astromart, both you and the seller are each given a "token" to provide feedback for each other. Most people use if the sale actually took place, but it is also possible to use that even if you did not end up buying from the seller (system does not track sale completion), but would like to provide feedback on the interaction.
3. After you have made the initial respond to an ad via astromart, subequent correspondence between buyer and seller is through email. No thread crashing, open bargaining and what not.
4. It is not possible to contact a seller outside of astromart unless you have dealt with the seller before and has his/her email address.
5. If you are not logged in, you cannot see the price of the item on sale.
I think it is a comprehensive and effective system, though it is not perfect. So far I have had very good experience buying stuff from astromart, and have never come across any seller with a negative feedback.
Interesting note also that recently ebay has removed the ability of sellers to provide negative feedback to buyers to prevent retaliative behavior from bad sellers.