Well from my perspective I agree. Definitely.
But this is the type of thinking that also got a well known VC [1] to not invest in airbnb even after PG strongly suggested that investment. (Which was documented in an email exchange back and forth that I can't easily find. Anyone?)
So with something like this maybe the idea is not to think "why won't this work" (which is an easy exercise) but "why would this work"?
I think a large part of it working would be the feedback system [2] and social constraints which is what makes many of these things possible.
[1] Fred Wilson
[2] As I learned many years ago the difference between buying a product from a company on ebay (which you can give seller feedback) and buying the same product from the same company off ebay (where the seller doesn't have as much motivation to do the right thing because you don't have the stick of feedback and reputation for motivation).