Well there you go! You've left out the most conspicuous bidders which would be private American citizens and corporations.
I see no need to further analyze your post if you didn't even see THAT one.
I didn't include it for several reasons. First off, our national debt exceeds GDP. Retiring our debt in four years would require 25 cents of every dollar earned by every single person in America. Yet a majority of Americans today pay no taxes. Secondly, if you confiscated the entire net worth of the 100 richest people in the world, you would be lucky to come up with $2.5 trillion in assets. And of those assets, less than 3% would be in the form of cash, meaning that they too would have to sell assets to raise dollars to bid on US assets.
The total assets of the top ten US banks comes to $10 trillion. And if every dollar of that was used to retire the debt, you would only be half way there.
There simply isn't anywhere near $20 trillion of cash capital available in this country. It would have to come from overseas where dollars are stacked up because of our trade deficit. Yet even with both domestic and foreign available, we are still printing money just to keep our government running.