Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 20

Sanskrit:

षष्ठं संवत्सरं तत्र चरित्वा रोहित: पुरीम् ।उपव्रजन्नजीगर्तादक्रीणान्मध्यमं सुतम् ।शुन:शेफं पशुं पित्रे प्रदाय समवन्दत ॥ २० ॥

ITRANS:

ṣaṣṭhaṁ saṁvatsaraṁ tatracaritvā rohitaḥ purīmupavrajann ajīgartādakrīṇān madhyamaṁ sutamśunaḥśephaṁ paśuṁ pitrepradāya samavandata

Translation:

Thereafter, in the sixth year, after wandering in the forest, Rohita returned to the capital of his father. He purchased from Ajīgarta his second son, named Śunaḥśepha. Then he offered Śunaḥśepha to his father, Hariścandra, to be used as the sacrificial animal and offered Hariścandra his respectful obeisances.

Purport:

It appears that in those days a man could be purchased for any purpose. Hariścandra was in need of a person to sacrifice as the animal in a yajña and thus fulfill his promise to Varuṇa, and a man was purchased from another man for this purpose. Millions of years ago, animal sacrifice and slave trade both existed. Indeed, they have existed since time immemorial.