Buyers cannot get negative feedback.
Do you mean the member is both a buyer and a seller? Sellers can get negative and neutral feedback.
Neutral feedback does not affect the rating. A member with 10 FB, one of which is neutral will still have a 100% rating.
How old is the feedback?
Feedback is no longer part of the rating after 12 months. A member who has five FB, three positive within the past year and two negative dated more than 12 months ago, will have a 100% rating.
Feedback is voluntary. Only about 40% of transactions get any feedback.
Feedback is not used by eBay to assess member accounts.
The two things to check with FB are patterns of complaint (poor quality, bad communication, slow shipping) and FB Left for Others, where the member's personality can be displayed.
