Anyone that wants to buy them on the open market. Since there would be a massive drop in demand for US bond markets, the bonds being sold would drop in price. It’s tough to say how far it would fall because you don’t know how many investors would be willing to buy what percentage of that $8 trillion. Any smart investor would know that amount of a selloff would take a considerable amount of time to recover from and also put the government in a really tight position. So why buy high? There’s always going to be people with more dollars than sense, so some will buy right away but large portfolios would wait and buy after an arbitrary drop. Say 30%. That would harm the dollar and would hurt the United States’ ability to sell new bonds to borrow money.

As for the one time sale part, europe would then look to other markets to reinvest the money in the short term. They wouldn’t want to hold that amount of cash unless they wanted to fund a massive infrastructure project(interstate system, rail network, military, housing, green energy, etc.) So I would guess that they’d look to the east and buy bonds in Southeast Asia/China. Its the only other market big enough. Then if you have trillions in foreign currency, you’ll want to invest in development to keep that investment safe. New trade deals would probably be discussed and foreign investment in developing technologies would help spur innovation. Suddenly the US would look much less attractive as a trade partner and investment test bed. The US could try to pull back similar investments but we invested heavily into non liquid assets. Manufacturing being a major one during the post war boom after WWII. It’d be very difficult to pull those assets quickly. Then to wrap this all up, Europe acquired those bonds by buying them. If they wanted to slowly sell off other investments and go back to buying US bonds, they could do so. Probably on the cheap. They make their money back and more as the US market would presumably recover…if we are able to depose regressive politics after an economic collapse.

Or just sell the $8 trillion in US bonds Europe own and crater the dollar overnight.