  Athey and Segal introduced an efficient budget-balanced mechanism for a
dynamic stochastic model with quasilinear payoffs and private values, using the
solution concept of perfect Bayesian equilibrium. We show that this
implementation is not robust in multiple senses, especially for at least 3
agents. For example, we will show a generic setup where all efficient strategy
profiles can be eliminated by iterative elimination of weakly dominated
strategies. Furthermore, this model used strong assumptions about the
information of the agents, and the mechanism was not robust to the relaxation
of these assumptions. In this paper, we will show a different mechanism that
implements efficiency under weaker assumptions and uses the stronger solution
concept of ``efficient Nash equilibrium with guaranteed expected payoffs''.
