It's not used in these forums, but if you buy or sell products in other forums (usually, technology forums), they use a feedback system called Heatware.
For example, I have a heatware of 39-0-0, which means 39 positive, 0 neutral and 0 negative feedbacks.
http://www.heatware.com/user_directory.php?searchQuery=ghostman
It would be nice if people started using it here.
Some people may also post their ebay ID to show their feedback there, but there's no way to associate that ebay ID with the respective user.
Otherwise, check a member's previous posts. If the member posts valuable feedback to others, it's a good sign that person is a more trustworthy member. Not a guarantee though.
BTW, most sellers do not want to go through 3rd parties, especially for low-ticket items. Extra costs, extra time and uncertainty about the trustworthiness of the 3rd party. In forum transactions, buyers pay and pray. I don't know about PayPal's insurance policy.