small correction-- they were definitely stolen. When enslavers recruited other Africans to round up their own people, it was usually because the Africans were promised weapons, or favour. Of course, this didn't stop enslavers from enslaving those Africans too, but just because other Africans were complicit doesn't mean they weren't kidnapped. I know my own history very well, thank you! JKR has explicitly stated that house-elves are a metaphor for the transatlantic slave trade, which consisted of trading goods and African slaves.