Models for online markets with reputation system
I would suggest you start by looking at


  C. Dellarocas. "The Digitization of Word-of-Mouth: Promise and
  Challenges of Online Reputation Systems". Management Science 49
  (10), October 2003, 1407-1424.


for a review of relevant literature and


  Friedman &amp; Resnick "The Social Cost of Cheap Pseudonyms". Journal of
  Economcs and Management Strategy. 10 (2), 173–199.


and


  "Sustainable Reputations with Rating Systems," 2011. Journal of
  Economic Theory, 146, 2, 479-503


for some models of online reputation systems that comes to mind. If I think of any others, I will update this answer.