Selling 40 billion will not collapse the treasury market. In the 3rd quarter this year, the US govt. is selling an additional 78 billion and I don't see the collapse anywhere in sight.
Some think China can sell at the same time, making our bonds worth so much less... meaning we'd have to increase their yield to sell enough to make ends meet.
It's nothing more than a blunt stick in they eye... making us pay more for our credit. But if they had $ trillions, it would be a sharp stick.