Oh, I wanted to point out one other fact about swapping tickets: It's illegal. But it is not clear that it is illegal to just place bets on both sides of the market.

Yes, it is prohibitted to be both long and short the same contract at the same time. It creates a false open-interest position. (i.e. It presents an illusion to the market that a position is open when in point of fact it is a "scam" transaction, it is misleading to participants in the marketplace.) As to the idea of swapping tickets, it ignores normal audit procedure. Trading procedure is as follows: (With thanks to Bruce M. Collins, V.P. Equity Arbitrage Group, Index Products Research, Shearson Lehman Hutton Inc. and James A. Schmidt, V.P. Equity Arbitrage Group, Shearson Lehman Hutton Inc.) A customer decides to hedge a position. The trader phones directly to the floor of the appropriate exchange and places the order with a floor broker. The floor broker executes the order on the floor, and phones a report back to the trader, where the order ticket is written and the customer account number is reported to the floor. The wire operator books the trade to the customer's account and sends a hard copy confirm to the firm's branch where the customer is located. On a nightly basis, the operation area of the brokerage firm will match all trade tickets to the hard copy confirms to verify the contract. The buy/sell, price, quantity, account number, open/close will all be checked for accuracy and commissions calculated for each ticket. In addition, operations will send details of the all the trades to the Clearing Corporation which then matches buy and sell orders across brokerage house inventories, and in the event of discrepancies adjusts contracts and dollars where necessary. Prior to sending the customer a confirm, a trading desk clerk will match trade tickets and reports with the hard copy customer confirms to verify the account. On properly matched trades the confirm is sent to the customer. If a correction is necessary, the clerk will adjust the trade and again verify all trade information on the confirms the next morning. Finally, the firm's margin department will settle all contracts. A check is issued on a sell to the customer, or on a buy the customer will deliver an escrow receipt from his bank. In addition, the margin department will assign operating requirements for any opening short positions and issue and margin call that may be necessary for new or existing positions. (This is performed on a nightly basis.) So, in short, yes a broker can swap tickets, however it does leave a full audit trail. Swapped tickets provide no anonymity. In this regard, the problem is the same as that of remailers. There are additional issues as well, money laundering usually involves laundering cash. Firms will not routinely accept cash deposits for margin. Funds must be on deposit, and freely available in order for the firm to settle it's daily accounts.