There is:
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Human trafficking forced labour debt bondage plantation slavery and many more
abolitionists would buy them at anti-slavery fairs to represent that they were against slavery.
to abolish slavery.
Slavery existed already in the earliest recorded times of history. For example, ancient Egypt and ancient China, which were thousands of miles apart, had slavery. Sometimes prisoners taken in war weren't returned but were kept as slaves; sometimes people with large debts had to 'sell' themselves into slavery. In other words, it seems that it isn't possible to say how, where or why the 'idea' originated.
In that time, it was perfectly acceptable to own slaves. Of course, slavery in that time wasn't the same as slavery in the US pre-1865.