sell down the river

Probably from the practice in the U.S., prior to the American Civil War, of trading in slaves who were transported via the Mississippi River:

Mark Twain (1885), chapter 42, in Huckleberry Finn: “"[H]e ain't no slave. . . . Old Miss Watson died two months ago, and she was ashamed she ever was going to sell him down the river, and said so; and she set him free in her will."”

Colin Woodard (2011), chapter 18, in American nations:"The least fortunate wound up on the sugar plantations of southern Louisiana and Mississippi, where it was sometimes profitable to work one’s slaves to death. Being “sold down the river” originally referred to slaves being sold by Appalachian people in Kentucky and Tennessee to downriver plantation owners in the Deep South.”