Daniel Singer runs a heating oil company in Elmsford, N.Y., and is a derivatives customer. In order to offer homeowners fixed-rate oil plans, he buys derivatives contracts. But since the trading system is not transparent, he can’t tell whether the prices he gets are fair or not.In theory, clearinghouses exist to safeguard the integrity of the multitrillion-dollar derivatives market. In practice, they also defend big banks’ dominance.