Simultaneous 2nd Price Item Auctions with No-Underbidding

We study the price of anarchy (PoA) simultaneous 2nd auctions (S2PA) under a new natural condition no underbidding, meaning that agents never bid on items less than their marginal values. establish improved (mostly tight) bounds PoA S2PA underbidding for different valuation classes (including unit demand, submodular, XOS, subadditive, and general monotone valuations), in both full information incomplete settings. To derive our results, we introduce parameterized property auctions, termed (gamma,delta) revenue guaranteed, which implies at least gamma/(1+delta). Via extension theorems, this guarantee extends to coarse correlated equilibria (CCE) settings, Bayesian (BPoA) settings with arbitrary (correlated) distributions. then show are (1,1) guaranteed respect bids satisfying underbidding. This 1/2 valuation, BPOA Moreover, (lambda,mu) smoothness combined guarantees (gamma+lambda)/(1+delta+mu). host such as tight 2/3 submodular (or XOS) valuations, overbidding Beyond establishing S2PA, assumption sheds light performance relative 1st auctions.