How was the demand for labor on the "slave frontier" met?

The correct answer is:

Slaves were sold in the South by owners in the border states.

The slave frontier´s expansion led to an increase in the slave trade. This western boundary passed through the middle of Georgia in 1790, and by 1830 it was stretching through Louisiana. By 1860 it was able to extend to Texas. This development in territory raised the number of slave states and the acquisition of land also presented an opportunity for planting crops that needed laborers.