(had not kad..sorry about that)
Actually before of the Civil war the worlds true super power had (The United Kingdom )decided to end as much as was possible the African slave trade.
To American credit there was an attempt to aid them in this cause and laws made ships sent.
See http://en.wikipedia.org/wiki/Blockade_of_Africa
This of course is in all likelihood the read the reason the CSA ended IMPORTATION of slaves from Africa as the British blockade being run by CSA flagged ship would not aid in the hope of the CSA to be recognized by the Crown.
A lot led up to the end of slavery and I was only addressing your original point. The founders of the US did not IMPOSE slavery on the country. They recognized that it was immoral and sinful, however they also recognized that the Constitution would not be adopted by all of the colonies if they addressed it immediately. However, they did present a constitution for adoption that provided the tools to future generations to resolve slavery.
Slavery existed long before the founders and although they were uniquely suited to the founding of the United States, they could not solve all evils in one fell swoop.
Per Article 1, Section 9 of the Constitution, Congress had the authority to outlaw the importation of slaves into the United States no earlier than 1808. In 1807 Congress passed a law to outlaw the importation of slaves into the United States. The law took effect in 1808. Any importation of slaves into the U.S. after 1808 was illegal.