Didn't think so.
Yeah, it would be impossible for it to happen. Even if one offered trillions upon trillions of dollars to "purchase" the United States, the three branches of the government would have to vote on it.
Not to mention the the outroar of people in the US who would retaliate if such a thing were to happen.
Sorry if I have my government wrong, I was asleep most of the time in my classes.