Complex sentence: They saw the abolition of slavery as a threat to their powerful new Southern market that revolved almost entirely around the plantation system which was furthered by the use of primarily African slaves, but also utilized destitute whites.
Split into simpler sentences: They saw the abolition of slavery as a threat to their powerful new Southern market. A market that revolved almost entirely around the plantation system and was supported by the use of (primarily) African slaves and destitute whites.