What if the gov held a T-bill auction, and nobody came?
The "Primary Dealers" always come.
That's one of the conditions of being a "Primary Dealer".
The problem will arise when the rest of the world stops pretending the Benbernank isn't then buying the T-bills from the Primary Dealers with freshly-printed dollars and starts pricing oil in something other than US dollars. That means that to buy oil we have to buy some other currency with dollars, and that's when the music stops.