I think having review threads directly linked in each marketplace thread would be a start, or even enough on its own. Just require brokers to put the link at the bottom of the first post in every marketplace thread. Simple enough, until a more technical solution is implemented (or not).
I don’t like thumbs up/down systems as they don’t allow for any concrete or detailed explanations for the ratings. What you value in a service is subjective to some degree. People also give positive or negative reviews for very questionable reasons sometimes. I can see people conflating issues with a dealership vs broker, etc.
I think if you’re going to give a rating on what is often a once-every-three-years transaction, you should give an explanation for your rating. Consider the recent/current situation. Simple numerical ratings don’t give any indication of time or trends. You can’t see a string of recent negative ratings in the sea of past ratings. The thresholds that people consider to be good or bad are also subjective.
They also tend to skew right (up) unless something is seriously wrong, by which point the damage has already been done. For example, ratings on food delivery services vs Google maps (and Yelp, although there are issues with this platform). The ratings seem to be cumulative, not unique, and so naturally skew favorably due to repeat customers. Obviously, buying a car is less of a daily occurrence, but the same concern applies to a lesser extent. Especially depending on how the system is implemented. If clients are allowed to give ratings based on their experiences before the system is implemented, then this becomes a real concern.
I think any type of numerical rating such as stars, thumbs, or percentage positive/negative should only be implemented if accompanied by written reviews. Otherwise, the numerical data becomes next to worthless as the numbers arguably become subjective.