It is not illegal for a U.S. citizen (or anyone else) to marry for money in the U.S. But from what I understand, it is a felony for a U.S. citizen to receive money from a foreign national to enter a sham marriage to facilitate the foreigner's immigration.
A felony charge has to be proved "beyond a reasonable doubt." Does this standard also apply to the foreign national who will have to face deportation?
And suppose there are complicating circumstances. For instance, the U.S. partner really wanted to be married to the other person even if money was a factor, while the foreigner did not. Or vice-versa. Would sanctions then be applied to only the guilty party and not the other?