SB 9.7.20

ṣaṣṭhaṁ saṁvatsaraṁ tatra
 caritvā rohitaḥ purīm
upavrajann ajīgartād
 akrīṇān madhyamaṁ sutam
śunaḥśephaṁ paśuṁ pitre
 pradāya samavandata
Synonyms: 
ṣaṣṭham — the sixth; saṁvatsaram — year; tatra — in the forest; caritvā — wandering; rohitaḥ — the son of Hariścandra; purīm — in his capital; upavrajan — went there; ajīgartāt — from Ajīgarta; akrīṇāt — purchased; madhyamam — the second; sutam — son; śunaḥśepham — whose name was Śunaḥśepha; paśum — to use as the sacrificial animal; pitre — unto his father; pradāya — offering; samavandata — respectfully offered his obeisances.
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.