Depending on when you sold the item, give them time to reply as not everyone is at the computer 24/7. If no payment or contact after 4 days open NPB dispute. You can not re list until the case is closed either by closing on day 8 or buyer paying. You can not leave negative for a buyer, and please do not leave positive with a negative comment, it is against policy, can be easily removed, and only increases the buyers FB rating.