Dupe. This paper is new, but the result is not.

The paper Betting on Permutations by Pennock, Chen, Fortnow and Nikolova already proves this.

Consider a race between n runners. Now consider a prediction market accepting bets of the form "X will finish before Y". They prove that the market maker/auctioneer problem is NP hard. Conversely, if markets are efficient, then they solve the market maker/auctioneer problem.