Feedback should be left when both parties participate in an exchange of items of specific value. Some examples outlined below of when and when not to leave feedback.
User 1 trades his Bioshock for User 2's Mass Effect. Feedback is left by both parties.
User 1 buys an Xbox 12 month code from User 2. Feedback is left by both parties.
User 1 gives User 2 a Playstation 3 and User 2 pays $20 for the shipping. Feedback is left for both parties.
User 1 gives User 2 a free 48 Xbox Live code. NO Feedback is left.
User 1 gives User 2 a free Playstation 3. NO Feedback is left.
User 1 trades a gmail invite for a qmail invite from User 2. NO feedback is left.