Was it the plantation owners who received the money made from selling their African slaves at an auction or someone else?

Yes, given the context.

Explanation:

As long as the plantation owner owned the slave and it was written down as his property in his inventory, he could do whatever he wanted with it and so if he sold it, since he owned it he would get all the profit.

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What is the purpose of the Necessary and Proper Clause?

It lists the federal government’s enumerated powers

Explanation:

The Necessary and Proper Clause1 concludes Article I's list of Congress's enumerated powers with a general statement that Congress's powers include not only those expressly listed, but also the authority to use all means necessary and proper for executing those express powers.