Article 1, Section 2, Clause 3
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.
Article 1, Section 9, Clause 1
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
Article IV, Section 2, Clause 3
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
That old reading style is difficult for me but still...where does it say you can buy or trade black people?
I know the Southern states wanted slaves counted in the census so they would be better represented (Representatives based on population count) and that is how they came up with the 3/5ths rule.
There were also indentured servants (white people) at the time and they didn’t want them to escape to another state and not work out their 7-10 year term of indentured servitude. But I guess I still don’t see where it’s okay to buy/sell black people.