We consider a network of traders intermediating exchange between a buyer and a seller. Traders face a private trading cost, a network characterizes the set of feasible transactions, and an auction mechanism sets prices. We investigate stable and equilib- rium network configurations. Bottlenecks in the trading network arise naturally in a free-entry competitive equilibrium and equilibrium markets organize into an asymmet- ric structure with many traders near the buyer and fewer traders near the seller. Large markets, while immune to bottlenecks, lack vertical stability in their organization and are vulnerable to collusive trading arrangements among neighboring traders. Stable markets supporting an arbitrary degree of intermediation between the buyer and the seller are shown to exist.