Hello,
To change those negative feedbacks to positive contact those sellers to send you a feedback revision to complete. You can also then change any low star ratings you may have left.
First always try and resolve matters with the seller before leaving negative feedback which can unnecessarily ruin a 100% good standing. Most sellers are only to happy to resolve a problem for you.
Another point to remember, many sellers will block a buyer from buying from them, who constantly leaves negatives.
