Straight from the FAQ (up above, in the little blue bar, where is says "FAQ")
What is the Trader Rating?
The Trader Rating option is your opportunity to rate your trading partner's performance and his chance to rate yours. You are rated in four categories:
1) Packaging
2) Delivery Time
3) Communications
4) Condition (as described)
These categories will be averaged together to create an overal trader rating.
You can give your partner a rating between 1 and 5, with 5 being the best and 1 being horrible. The trader ratings are confidential, so no one will know what score you have given.
Trader ratings are based on a minimum of ten ratings. For every 30 ratings, the highiest and lowest scores will be dropped. The Bench's software automatically recalculates the trader ratings every evening.
A member's individual trader ratings appears in their profile page.