The Itrader system is still present but not super user friendly.
To leave feedback;
First go to the for sale thread and copy the link
next, go to the person you want to leave feedback's post and click on the number next to their "feedback score" (even if it's 0), this will get you to their itrader screen
On the right near the top will be a link to leave feedback, click that
You will need to enter the link to the selling thread, and enter your feedback.
Keep in mind, if you give them negative feedback there is nothing stopping them from leaving you negative feedback in response. It's far from a perfect system, but as long as we don't have a BRS judge or jury we're pretty limited......