If you go to the marketplace site, and click on the member's name.
You'll see a drop down menu. Select "view profile"
Once there, you'll find a tab "feedback score", click that.
Here you'll find a link that says "leave feedback" click that.
You will need the URL of the marketplace thread. Identify yourself as either buyer or seller.
Rater your overall experience, & leave a comment.
Hope this helps,