This link answers your question.
Note: Sacrifice in the text needn't mean cutting of throat as modern so-called priests do. Sacrifice then meant rejuvenating of life through mantras.
But the point is, whether it existed or not, our conception about slavery might be wrong. Vineet's comment is important to note in your question. There is a huge difference between modern conception of slavery and the spiritual conception.
Then, air was clean and sex was considered dirty. Today sex is considered clean and the air is dirty.
In the spiritual sense, to accept the position of a servant (slavery) is a very high position. Rather, Shrila Prabhupada, founder of International Society for Krishna Consciousness said - the more one is a servant, the better he advances in spiritual life (dasanudas).
This is called paramparā system. You have to learn how to become
servant of the servant of Kṛṣṇa. The more you become in the lower
position—servant, servant, servant, servant, servant, hundreds times
servant, servant—then more you are advanced. Here in this material
world everyone is trying to be master of the master. Just opposite.
And the spiritual world, the endeavor is to become servant's servant.
This is the secret.
[Lecture on BG 2.2 -- London, August 3, 1973]
In material world, our experience of being subserviant to a master is very bitter but in the spiritual world, there is no difference between the master and servant. Because there, everything is Absolute.
That's why we see Krishna, although The Supreme Personality of Godhead, wants to accept the position of Arjuna's chariot driver, which is a very humiliating service if you know the ways how the chariot driver is guided to turn (through kicks). So the modern understanding of slavery is to be rectified.
Everything in those times was aimed at making spiritual progress. A shudra would be engaged in the service of a brahmana so that they can make progress towards the brahmana varna in the later births gradually. A good example is Srila Narada Muni, who was a son of a maidservant.
So I hope there is a good understanding of trading then and today. Today's trading is completely centered around gratifying one's own senses. Then, it was all centered around Vedic conception or pleasure of the Supreme Lord Krishna or other demigods.