Taking another page out of Google's book, we've recently been seeing merchant ratings as an extension to Bing paid search ads:
Although its ads are also run through Microsoft's adCenter, ratings are not appearing on Yahoo for the same advertisers on the same queries:
In general, almost any ad extension that gives more real estate to a paid listing should improve its click-through rate, so this should be a welcome sight to advertisers. Besides simply taking up space, seller ratings also lend credibility to the sites for which they trigger and give confidence to users, particularly when the ratings only appear if they meet a certain threshold. With Bing still struggling to keep pace with Google's ad traffic growth, this should be a step in the right direction for them.