The way I see it, the retail clients connect to a broker platform (IB, ..) and it is there that the above scenario can happen. The broker has an account in the

The way I see it, the retail clients connect to a broker platform (IB, ..) and it is there that the above scenario can happen. The broker has an account in the exchange and uses this account to offset the global position of all his clients in his system. Using this information is generally possible for any intermediary (bank, broker, ..) To be clear, I am not saying everybody does it but I think that they can, they want to and regulatory oversight in the past could not even keep track of the total stock of a broker (see MF Global).