Before revision:  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:  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 . For the general problem, we prove that it is NP-hard to find a constant-factor approximation .