The primary justification given for the slave trade was economic; the South's economy was wholly dependent on it. Any attempt, then, to abolish it would lead to the collapse of the Southern economy and the society on which it was based. In reality, however, the slave trade held back the...
Southern economy, preventing it from developing any kind of significant industrial base. This put the South at a distinct disadvantage in the Civil War, as the Northern economy was much more modern and diversified.
Strange as it may seem, many advocates of slavery used religion to justify their position. A narrowly literalist interpretation of Scripture formed the basis of much pro-slavery argument. Verses from the Bible were selectively quoted and distorted to make it seem that God himself had ordained the peculiar institution. At a time in the United States when Bible-based faith was widespread, this was a potentially powerful argument. If God has given us slavery, then who are we to defy His will? An economic activity such as the slave trade is always much more difficult to challenge if it has the stamp of divine authority.
An even more persuasive argument to many at the time was that black people were racially inferior. Even the vast majority of those opposed to slavery believed this, backing up their racial prejudice with pseudoscience such as phrenology, which sought to demonstrate biological grounds for the alleged inferiority of certain races. If black people were biologically inferior, so the argument went, then it was perfectly acceptable to treat them as such. Though this did not necessarily mean that they should be enslaved, it did at least lend some support to the argument that slavery and its associated trade were not just a manifestation of divine order, but of the natural order as well.