Look, if China sells those bonds, then China's going right down with the US and the rest of the world economy. There will be turmoil in the world market. It's not as if this is a one way thing. Who will the Chinese sell those cheap manufactured goods to?
You can look at it from another angle: Right now, they're selling their goods to someone who is never going to pay them. It is impossible to be ignorant of this. Nothing on the world is worth less than a piece of US money or an US cheque. Well, maybe an US credit card.
The only reason why these things are nevertheless "worth something" is because a) the USA are bullying everyone with wars and embargos (but a country the size of China needs not be afraid of that) and b) because there is a silly notion of "trust" and "handing off debts" in this world. Meaning that if someone tells you that they promise to pay, you pretend to trust their word, because there is likely some bigger fool who will believe you and give you money (which, itself, isn't backed by much but a promise) in return for a bond on those debts.
Who would buy the bonds/certificates if China threw them at the market? Nobody, this isn't necessary at all. Assume China throws a billion US dollars (or two) on the market. US dollar rate will go down to zero in 10-20 seconds. Automated traders will catch up a significant drop and immediately sell everything they have to avoid being the last fool to sell, amplifying the effect further and finally causing a crash that nobody can absorb (this happened in March 2000, although on a much smaller scale). The USA will be incapacitated within minutes. There's not enough money in the world to stop such an avalanche, so dollars and anything related to them will be toilet paper in minutes.
People will revolt in the streets. But also, institutions like law enforcement and the military are paid in dollars, and they wouldn't risk their butts knowing it's just toilet paper they work for. It's hard to start a war when your soldiers no longer follow you, when there's burning cars in the streets, and when your people are knocking at your door wielding pitchforks. It is also a very unlucky condition when the people who drive the heavy tanks and wield the machine guns are unhappy. You
really want to be able to pay them, because when you can't you're totally fucked.
Would this affect the rest of the world? Certainly. Would this destroy a few other countries? Probably, at the very least it would destroy every major bank in Europe (who have significant enough dollar investments so they wouldn't recover). Would this destroy China? Unlikely. It would certainly be "uncomfortable", but not necessarily much more.
There's this saying "I care as much as if someone topples a sack of rice in China". This is reciprocal. They're big enough so they need not care, and unlike everyone else,
they have production lines for everything. We can't do without China, but they can do without us. And they know that.
Heck, you wouldn't even be able to buy a T-shirt if China wasn't making them. Everything you need for your daily life comes from China. Including 95% of the parts in the computer you're using to read this (the other 5% are probably from Taiwan...).
Without them, we're simply unable to survive as it is. But that's not true for them.
Nobody will want to risk this. Nobody.
And that's why I'm sure there will be no war. The USA may shake an angry fist and maybe they call back their ambassadors (oh how very sorry the Chinese will be about this serious punishment!) but in the end, if China chooses to take those isles, then that will just be it.