Let's keep this SIMPLE and easy on the Admin here:
1. Start a Child Board in Classifieds titled "Feedback".
2. Each person buying or selling junk in the Classified areas creates a post with their Username as the title of the thread.
3. Put a link to your Feedback thread in your signature, or in the post you make when you buy/sell in the Classifieds. That way you can check out the feedback from sellers/buyers previous transactions. People who are new can get a feel for the seller, or we can be extra careful with new sellers.
4. After each successful transaction, both parties can go to the other persons thread and post Feedback.