The free market, remember, is only efficient under certain circumstances, particularly: transaction costs must be low and there must be few barriers to entry.

AirBnB et cetera lower the transaction costs and remove barriers to entry.

Easy discovery of price information is also important to capitalistic markets. Of course, AirBnB helps there as well.

I know what you mean by "rent seeking", but in this context it sounds funny because what Airbnb is all about is allowing everyone to, literally, rent seek.

i suppose rent seeking isn't inherentely bad, but over charging for rent is. And you can over charge rent if the barrier to enter into the rental business (i.e., become a landlord) is high. AirBnB actually lowers the barrier to entry for rent seeking, and thus, make rent seeking fairer over all as theres more competition.

To take it slightly off topic, the banking and finantial sector could use some lowering of barriers to entry. Banks aren't bad, but banks that are too entrenched are. What sort of lower barrier could you set so that there could be 'mom and dad' banks?