Earlier quoted context omitted.
I would love it if you could elaborate on this. Since most exchanges don't do anything before they received money from buyer, what would they need the money for?
The huge fluctuations in the price of BitCoin probably cause problems. Taking an order, locking the price in, and waiting a few days for the transfer gives BitCoin time to drastically change in value.
Well, at the time, I hadn't planned to make up the difference at Coinbase, but that was my first plan. The due date of the payment would have been after the closing date of Coinbase transactions on that date. I could probably still buy more coins before they were due, if I was still winning the auction for 6BTC.
But the real plan? SatoshiDice. I'm not talking double, I had most of the BTC needed for my bid, and the difference between what I had bid and the next bidder (split the difference) was even smaller.
So, if I just need to win 0.9BTC, it's pretty easy to do that at SatoshiDice with 4.5BTC bankroll. In fact I won 0.7BTC yesterday, after someone outbid me. I could have made it up in cash at Coinbase, but then there would be no Christmas.
This however is a small auction for another speculative asset between friends. I am not a financial services company! I can't imagine the size of the spreadsheet required to predict whether Coinbase will be able to meet all of its obligations to every customer at each point in time given the possibility of price fluctuations. Certainly a harder problem than "can I maybe win 0.9 at SatoshiDice."
But certainly also, within striking distance of solvable.