Just because the only way I think it will take off is if the ratings are displayed on the actual craigslist posts.
I would totally use that as a plugin. Why do you think that couldn't/wouldn't take off?
Well, I was counting on the viral aspect where you saw a rating and said, I want one of those. My gut says that most people who do that will not want to install a browser plugin. Even if I made one for all possible browsers.
It's also hard for it to spread through the plugin directories because the people who download it would be too geographically fragmented.