For starters, less than 5 stars isn't necessarily a "negative" rating. 3 or 4 stars indicate Neutral or Satisfactory ratings and are not considered negative. If you were left a truly negative rating, you should take a step back and ask yourself if the buyer or seller's complaints are valid. Its possible that for whatever reason the auction didn't go well, and you deserve the rating. You should leave an appropriate comment and rating for the other person; this doesn't mean leave them a negative rating and comment as well. If they held up their end of the bargain satisfactorily, you should leave them a rating and comment that indicates as much.
If you feel as though you received the negative rating or comment when the transaction was not problematic in the least, file an abuse report (through the members' area) or contact the site administrator.