You will first need the URL thread address for the item that was bought or sold. (the URL is the http://wwwblahblahblah that appears in your browser at the top when you are in that thread.) Copy that as you will need to paste it in a minute.
Click on the persons name.
Click on contact info.
Click on Feedback
Follow rules there for either buyer/seller
Paste the URL in appropriate place.
Leave comments.
Thanks for using the Feedback. The more we use it the better it will be.
Any further questions just give me a p.m. shout.