That would have to work on a star-rating. Based on the number of asterisks in the word that eBay decides is too rude to view...
Tw*t = 1 star (lowest, mildest negative rating)
Wa**er = 2 star (could do better)
F***er = 3 star (not good at all, could do worse)
**** = 4 star (Utterly despicable)
*****bag = 5 Star (Well, you WERE warned...)