[Desron13]

By the way. The phrase “sold down the river” refers to African blacks taking slaves in tribal warfare and selling some of them to mostly Arab slave traders who would then sell them to the Europeans. Most travel in Africa was via river traffic. Thus the Sold down the river phrase.

[EternalVigilance]

Hmmm....I always thought that referred to the practice of sending troublesome blacks further South as field hands.

[Sherman Logan]

The phrase “sold down the river” refers to African blacks taking slaves in tribal warfare and selling some of them to mostly Arab slave traders who would then sell them to the Europeans.

Nope. It's a phrase as American as apple pie, referring to selling slaves from the Upper South down the Ohio and Mississippi rivers. First reference 1837.

Slaves in the Deep South were often worked much harder and treated more brutally than those in the Upper South, especially in the sugar plantations of LA, and slaves were terrified of being sold down the river.

http://www.phrases.org.uk/meanings/326500.html