uhh simply because he don't have them. He had just acc on some exchange, some BTC on stock and in case of needs, he will transfer fiat to exchange and sell it again.
you know what? at 310$ I can sell you unlimited amount of bitcoins..)
Thats it. He simply will buy those bitcoins cheap when someone will buy for that price. And he only has to fund the escrow once the trade starts. Its not that he have those coins yet.
If he would own them at the moment, and he thinks he has to sell them, then i would assume he will sell fast in order to lower its risk that the price will drop.