The 20-year-old woman was arrested during an undercover operation following a tip-off from a member of the public that a baby was being sold on the website Gumtree, the Hawks police unit said.
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She was due to be charged later in the Pietermaritzburg Magistrates Court.
"What we have already ascertained is that the baby is not hers and the baby is now with the care of social workers," Hawks' spokesman Hangwani Mulaudzi told Eyewitness News.
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"We are trying to ascertain where this baby comes from and what exactly were this woman's intentions." The woman's identity has not been made public, so it was not possible to contact her for comment.
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