Yes, you could do this via the wiki* quite nicely, though it is open to abuse (dealers submitting their own reviews, or deleting bad ones - though this is easily tracked) or possibly dealers posting poor reviews about rivals. There should be clear instructions that any reviews should be factual, since the poster may be liable to legal action from the dealer if it is not factual and business levels are affected.
The folder tree format of the wiki might help navigation too, if it looked like:
Main Page link to "Dealer Reviews" > List of Counties > Alphabetical list of dealers > reviews.
* The wiki is there, but not properly launched yet, since I am writing articles bit by bit. But have a play at
www.civinfo.com/wiki