Twain does by way of insinuation and implication discuss and define being
sold down the river in PUDD'NHEAD WILSON.  The character Roxana does
react to the threat of being sold down the river by Percy Driscoll.  He
says:

        "I will sell you DOWN THE RIVER!"  The narrative continues,
        "It was equivalent to condemning them to hell!  No Missouri negro
doubted this.  Roxy reeled in her tracks and the color vanished out of
her face;  the others dropped to their knees as if they had been shot;
tears gushed from their eyes, their supplicating hands went up, and the
three answers came in the one instant . . . " (12)

In the following chapter (Chapter 3), Roxy agonizes over the terror of
the threat of being sold down the river.  And she subsequently makes the
switch of the babies .

Also, later in the book in Chapter 16, her son "Tom" does "forge a bill
of sale and [sells] his mother to an Arkansas cotton planter for a trifle
over six hundred dollars.  He did not want to commit this treachery, but
luck threw the man his way and this saved him the necessity of going up
country to hunt up a purchaser" (81).  In the same paragraph Twain  goes
on to discuss some of the implications of being sold down the river.  He
equates it with such "treachery as . . . treason to a mother" (81).

I'm sure it's also discussed in the same  indirect way in HUCK FINN as
well, possibly even in TOM SAWYER--but never directly.  Of course, that
wasn't Twain's way.  He was most effective because of the implications
and irony of the situations in which he placed his characters.

You may also want to look in some of the Slave Narratives, such as by
Harriet Jacobs or Frederick Douglass.  I know when I have taught
PUDD'NHEAD and the slave narratives, there are many parallels.

Hope this helps,

Carolyn Richey
SDSU
PS:  The quotes are from the Norton Critical Edition