We study markets where the characteristics or decisions of certain agents are relevant but not known to their trading partners. Assuming exclusive trans- actions, the environment is described as a continuum economy with indivis- ible commodities. We characterize incentive constrained eÆcient allocations as solutions to linear programming problems and appeal to duality theory to demonstrate the generic existence of external e ects in these markets. Because under certain conditions such e ects may generate non-convexities, random- ization emerges as a theoretic possibility. In characterizing market equilibria we show that, consistently with the personalized nature of transactions, prices are generally non-linear in the underlying consumption. On the other hand, external e ects may have critical implications for market eÆciency. With ad- verse selection, in fact, cross-subsidization across agents with di erent private information may be necessary for optimality, and so, the market need not even achieve an incentive constrained eÆcient allocation. In contrast, for the case of a single commodity, we nd that when informational asymmetries arise after the trading period (e.g. moral hazard; ex post hidden types) external e ects are fully internalized at a market equilibrium.
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