Why slavery was good?

Economically, slavery was a boon to many. First, African tribesmen captured other blacks and sold them to European slave traders. The capturers benefited financially from these transactions by receiving gold, silver, weapons, etc...

The slave traders loaded up shiploads of slaves and sold them to slave owners, receiving money in return.

The slave owners benefited from slave labor for the life of the slave without having to pay a salary.