eBay has now completely changed the feedback system. Sellers can no longer leave negative feedback for buyers. Only buyers are allowed to leave Negative or Neutral feedback comments. Sellers only have the option of leaving Positive feedback.
Buyers who have a problem with a product or seller should contact the seller before leaving feedback, so that the seller can try to resolve the situation. Most sellers are very helpful and will want to help, but leaving feedback before the problem is resolved is unfair and the seller may then not want to help.