Problem Description
Internet platforms and permissionless blockchains let any participant spin up unlimited Sybil identities at negligible cost. Traditional dominant-strategy mechanisms such as Vickrey–Clarke–Groves (VCG) guarantee that reporting your preferences truthfully is the best strategy. However, in general, this is no longer true in the presence of Sybils. For example, in combinatorial auctions with two or more heterogeneous items, an agent can bid across fake accounts and pay less. The research question is therefore:
Under which structural conditions on the agents' valuation space does the welfare-maximizing VCG mechanism remain Sybil-proof, so that efficient outcomes are still attainable in fully anonymous environments?
This question is fully answered in some well-studied settings, most notably combinatorial auctions [1]. It remains open for markets with one or multiple divisible goods, for the provision of public goods, or markets with semi-fungible assets [2].
