Still, in order for MtGox to facilitate a trade they must have an account per user, that will hold a certain amount of btc and fiat. So, say I wanted to buy 1000 EUR worth of bitcoin, I arrange a bank transfer from my bank to MtGox. This may take a few days, and then they have my money in their account. Now, maybe during that time, the BTC/EUR exchange rate went up, and I decide to wait for the price to drop. In that case, the 1000 EUR will be sitting in their account until I decide to buy btc (or decide to cancel, and transfer it back). If there are enough people like that, they will have quite large sums of money in their account.