  Fisher markets are those where buyers with budgets compete for scarce items,
a natural model for many real world markets including online advertising. A
market equilibrium is a set of prices and allocations of items such that supply
meets demand. We show how market designers can use taxes or subsidies in Fisher
markets to ensure that market equilibrium outcomes fall within certain
constraints. We show how these taxes and subsidies can be computed even in an
online setting where the market designer does not have access to private
valuations. We adapt various types of fairness constraints proposed in existing
literature to the market case and show who benefits and who loses from these
constraints, as well as the extent to which properties of markets including
Pareto optimality, envy-freeness, and incentive compatibility are preserved. We
find that some prior discussed constraints have few guarantees in terms of who
is made better or worse off by their imposition.

Computer Science and Game Theory;Computers and Society