From "America! A Concise History" by Rorabaugh and Critchlow:
"...Another compromise provision gave Congress the right to abolish the African slave trade in 1808. In fact, few new slaves entered the United States after 1787, every state except South Carolina banned the overseas trade, and Congress did abolish this trade in 1808."
Some southerners were ignoring the ban and running slaves into southern ports well into the late 1850's.