There are two kinds of feedback on Bottle-Spot. Each pertains to the type of listing you are giving feedback through:
When you successfully bid and win an auction, you will have the opportunity to leave feedback for the user. This includes a short message about the user and your experience, as well as a rating. This kind of feedback can only be left by winners of an auction.
You can access the feedback system via your "My Account" page when logged in to the site.
Listings themselves don't provide any mechanism for feedback, but you can leave feedback on a user by clicking the five stars next to their name on the listing. In this case a one star rating is considered poor, and a five star rating great.
Keep in mind that anyone can submit a rating, so over time the rating should be somewhat representative of that user, but when the user only has a few ratings it may be skewed.