When Tom returns him to his wife and tells her all about his meeting with Old Scratch, we learn his reason for not making a deal. According to the text:
However Tom might have felt disposed to sell himself to the devil, he was determined not to do so to oblige his wife; so he flatly refused, out of the mere spirit of contradiction.
In other words, Tom does not make a deal with the devil because he does not want to please his wife. It has nothing to do with the fact that he does not want to sell his soul to the devil.
Tom's reason reveals an important aspect of his character and of his marriage more generally—that is, Tom and his wife are in permanent conflict, and Tom is happy to live without the pirate treasure in order to spite his wife.
His wife, however, is determined to spite Tom. As such, she sets off for the old Indian fort, determined to seek out Old Scratch and make her own bargain with him in return for the treasure. This shows that both Tom and his wife are each as spiteful as the other.
