Tom doesn't become a slave trader. What action does he take instead?
In the forest, when Tom Walker makes an agreement with Old Scratch for wealth, he is unwilling to go so far as to sell human flesh, refusing to be a slave trader. Instead, he becomes a usurer since the
Devil [was]...extremely anxious for the increase of usurers, looking upon them as his peculiar people
and it appealed to Tom. So Tom Walker opens a broker's shop in Boston the following month. In a short while, Tom begins lending money for "a good consideration." Then, when what was known as a speculating fever broke out in the region, people were dreaming of making quick fortunes. Walker's door opens to "the needy and the adventurous," speculators, merchants with poor credit, and tradesmen. Showing no sympathy, Tom exacts the worst interest from the neediest applicant. He acquires mortgages and bonds and soon became rich. But Tom begins to worry about the next world. "He became, therefore, all of a sudden, a violent churchgoer." He purchases the largest Bible he can find and lays it on his desk. Of course, he moves it to the side as he drives "some usurious bargain."
If I remember correctly, Tom becomes awfully religious. He begins to go to church believing that he can "chase the devil" from him. He keeps a large Bible on his desk and carries a small one in his pocket everywhere he goes. He also becomes very judgmental and keeps track of others' sins to show he is not as sinful as others.