The delegates at the constitutional convention ended the slave trade effective immediately following the ratification of the constitution.

This is false. The Constitution was ratified in 1788 (the first state ratified it already in 1787, Dezember 9th.

Slavery was outlawed only in 1865, with the Thirteenth Amendment

It is not true that the delegates at the Constitutional Convention ended the slave trade effective immediately following the ratification of the constitution however, the constitutional convention was afraid that the new administration will try to stop the slave trade and may just abolish it for good. In order for the slave trade not to be abolished, the Northern agreed made a deal not to stop the slave trade and they allow that the slave be counted as three-fifths of a person for the purpose of representing it in the Congress.