Today, the New York Times featured the research project that I worked on while at the University of Michigan. And carried a nice picture of my advisor, Prof. Paul Resnick. It's great to see the research featured in NYTimes, even though the only quote from Paul was "The data are a researcher's playground"!
Online markets are certainly growing in size, and also maturing as they grow. Economic studies of these markets are certainly vital to the field. Do check out the website: Reputations Research Network.
There are several excellent research papers listed at the site. For a gentle introduction, read the Reputation Systems article.