ow does Harely defend his position as a trader?

Haley refers to an old Bible passage that he interprets. He says God cursed Cain's children to be black.

The young man who had spoken for the cause of humanity and God before stood with folded arms, looking on this scene. He turned, and Haley was standing at his side. “My friend,” he said, speaking with thick utterance, “how can you, how dare you, carry on a trade like this? Look at those poor creatures! Here I am, rejoicing in my heart that I am going home to my wife and child; and the same bell which is a signal to carry me onward towards them will part this poor man and his wife forever. Depend upon it, God will bring you into judgment for this.”
The trader turned away in silence.
“I say, now,” said the drover, touching his elbow, “there’s differences in parsons, an’t there? ‘Cussed be Canaan’ don’t seem to go down with this ‘un, does it?”
Haley gave an uneasy growl.
“And that ar an’t the worst on ‘t,” said John; “mabbee it won’t go down with the Lord, neither, when ye come to settle with Him, one o’ these days, as all on us must, I reckon.”
Haley walked reflectively to the other end of the boat.
“If I make pretty handsomely on one or two next gangs,” he thought, “I reckon I’ll stop off this yer; it’s really getting dangerous.” And he took out his pocket-book, and began adding over his accounts,—a process which many gentlemen besides Mr. Haley have found a specific for an uneasy conscience.