Feedback is left in the marketplace after a sale. If you respond to a forsale post, a leave feedback tag appears in your post for the OP. Same with your post, as the OP can tag your post to leave feedback.
Any system that requires a full page of examples and explanations is pretty poor. It should be extremely simple and intuitive that doesn't require any instructions at all. Just add a feedback button on everyone's profile page and "bingo" it's all done.