This Nov 2012 blog post by Bill Gurley of Benchmark Capital is very insightful, and cites some great examples as well:

All Markets Are Not Created Equal: 10 Factors To Consider When Evaluating Digital Marketplaces
http://abovethecrowd.com/2012/11/13/all-markets-are-not-created-equal-10-factors-to-consider-when-evaluating-digital-marketplaces/

Also, see the deck here which takes Bill Gurley's ideas above and translates them into an evaluation framework:
http://www.nirandfar.com/2013/03/making-a-marketplace.html