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. We thus look at special cases and provide an approximation algorithm that finds an \varepsilon-suboptimal mechanism when it is not too hard to predict the possible type of buyers who will make the purchase  .
After revision:  2. For the general problem when the seller faces buyers of different types and only knows the distribution of their types, we provide an approximation algorithm that finds an \varepsilon-suboptimal mechanism when it is not too hard to predict the possible type of buyers who will make the purchase .