Before revision:  2. For the general problem when the seller faces buyers of different types and only knows the distribution of their types, it is NP-hard to find a constant factor approximation.
After revision:  2. For the general problem , we prove that it is NP-hard to find the optimal mechanism. (2) When the seller faces buyers of different types and only knows the distribution of their types, it is NP-hard to find a constant factor approximation.