We don't think much about the fact that King Benjamin made slavery illegal. But it is interesting because a form of slavery was clearly spelled out by the law of Moses. Thus, even with the law of Moses there was continuing revelation. The law of Moses declared that a man would be sold to cover debts he couldn't pay. In fact, I think it is Hosea who buys his unfaithful wife who abandoned him when he later finds her being sold to cover her debts. The law of Moses even laid out laws governing this slavery or forced servitude, i.e. a person could buy their freedom, and every 50 years there was a jubilee and all such people were freed.
However, in the Book of Mormon King Benjamin abolishes slavery, and we later learn that when a man can't pay his debts, he isn't sold to cover those debts as the law of Moses lays out. King Benjamin, who was also a prophet, had made slavery illegal. Thus, instead of what the law of Moses lays out, in the Book of Mormon we read that a man who can't pay his debts is stripped and cast out as a thief and a robber.
So while the Nephites lived the law of Moses, it was still a law of Moses that was subject to continuing revelation through a prophet of God.