Feedback is a tool to help both parties make decisions on who to do business with. 

Some sellers have taken the feedback process to a near "art form" of coercion. You will see things like "Will leave feedback after you leave feedback" or "If you leave negative for me, I will leave negative for you".  When I see these types of remarks, I avoid these sellers.

When a buyer pays for the product, they have completely fulfilled their side of the transaction. They deserve feedback from the seller stating that the transaction has been paid for, not being forced to give a positive feedback before they receive the same in return.

As a buyer, you have responsibility as well.  If you leave feedback before you receive and examine the product, you weaken the system. Do not leave feedback before the product meets your expectations, or you have received satisfaction from the seller. Understand how to use the e-bay system to help you when you get into a conflict with a seller.

Feedback is not a mutual pat on the back system, or reward and punishment system. It's a system that, when used correctly, will allow everyone to make an informed decision on who they want to do business with, and with what measure of risk.