You can leave a positive score any time, this is how
Read the post relating to sale or offer that was made (suggest in the Classified's section), ie http://www.avforums.co.za/index.php/topic,59109.0.html
Click on the name of the poster. Another window opens
In the centre, second from the bottom, is a prompt "Submit feedback for xxxxx"
Click on this
Chose buyer, seller, or trade
Choose positive or negative
Write some suitable words
For the topic URL, you can leave that blank or copy and paste the link above
Click "Submit feedback"
The poster then receives an email telling him you have submitted feedback