Navigate to the profile of the person you want to leave feedback for, click the Feedback Stats tab and press the Add Feedback button.
When positive feedback is left by a member for a particular thread, the recipient is notified. That feedback does not add to the recipient's trading total until they return feedback for the same thread.
Then, both traders' totals are updated at the same time.
This feature encourages both parties involved in a trade to leave feedback for one another.