Upper South states were indeed selling slaves into the deep South and to the West, and this not only weakened the institution, but allowed for the development of a more diverse economy. While slavery was still very important in Virginia until emancipation, there were also many more small farmers who grew diverse crops there, particularly in the Shendandoah. The state, particularly Richmond, featured some of the only heavy industry in the future Confederacy as well. I would add, however, that it was actually in the Upper South that tobacco was important, not the Lower South, where cotton was central to the economy.
Slavery would have died a quicker "natural death" in the upper South. In the lower South, the economy still centered around cotton which was very labor intensive. Slaves were being sold from the upper South where there was less demand to the lower South where there was more.