Feedback is very important to S&S and the sellers/bidders! It's good for you to know who is a good seller, and who is a good buyer, so leaving feedback whether good or bad is a great thing to do!
When you log into your members area you will notice on the left hand side there is a column of options.
Under Reputation you will find, Leave Comment.
This is where you will leave your feedback.
Click on this and you will see a list of the auctions you have either purchased or placed, in the right column of your auction list you will see CONTINUE. Click it.
This will bring you to the feedback area. You do two things when leaving feedback, first give the other person a rating of 1-5, 5 being excellent 1 being poor. Then you can leave comments in the text area.
Click Submit and you're done!
Congrats you have submitted feedback AND helped to improve S&S!