There were many services to the large planters plantations that were supplied by local merchants. While the larger plantations were self sufficent for the most part, the smaller ones weren't. And even the larger plantations found it more economical to buy some products from outside the plantation rather than buying a slave and spending the monies to set up certain trades on their plantations like blacksmithing and shoe and harness making. It became cheaper to buy things needed on the plantation than to invest the time manpowers and expense of equipment to make the same things on the plantation itself.
For example cloth making. by the time you have set aside the land necessary to grow the cotton and flax or for the pasture for Sheep for their wool for the materials to make the cloth. Then you figure the time of the slaves that is necessary to gather these materials, process them, spins them, weave them into cloth and then make the cloth into clothes for the slaves, you as a plantation owner have invested quite a lot into the clothing of your slaves when they could be doing more productive things.
So owning a slave wasn't "Cheap" from that perspective and plantation owners, as farmers do today, were always looking for a way to cut costs. The large Plantation could do all of these things themselves that is true. But that wasn't the case for the smaller owner/operations. And as their needs were filled by local tradesmen the larger plantations owners began to use these service also because it became cheaper for them to do so, and they could more effectively use their own work force, even in some cases even reduce their workforce.
Another things is that Cotton was no good sitting in a warehouse, so shipping was attracted to the Plantations. That created jobs. With shipping came more varied types of business. Deck hands and teamsters for example had to eat too but they didn't grow their own crops. There were plenty of support businesses which surrounded the Plantations and with those businesses came the merchants and vendors to supply the needs of those who did not live on the plantations.