  We provide simple and approximately revenue-optimal mechanisms in the
multi-item multi-bidder settings. We unify and improve all previous results, as
well as generalize the results to broader cases. In particular, we prove that
the better of the following two simple, deterministic and Dominant Strategy
Incentive Compatible mechanisms, a sequential posted price mechanism or an
anonymous sequential posted price mechanism with entry fee, achieves a constant
fraction of the optimal revenue among all randomized, Bayesian Incentive
Compatible mechanisms, when buyers' valuations are XOS over independent items.
If the buyers' valuations are subadditive over independent items, the
approximation factor degrades to $O(\log m)$, where $m$ is the number of items.
We obtain our results by first extending the Cai-Devanur-Weinberg duality
framework to derive an effective benchmark of the optimal revenue for
subadditive bidders, and then analyzing this upper bound with new techniques.
