Police were tipped off that the woman, aged 20, had offered her son for sale on the popular advertising website Gumtree.
"We met her and there was an exchange of money with an undercover police officer," Hangwani Mulaudzi, spokesman for the Hawks special crimes investigation unit, told AFP.
"She gave us the money and we arrested her directly."
The woman appeared in court in Pietermaritzburg, KwaZulu-Natal province, on human trafficking charges, and her case was adjourned until October 26.
"The team immediately removed the ad and blocked the user's account. (We) escalated the issue to the corporate office that engages law enforcement on suspected criminal cases."
The child was in good health and has been taken into care by social workers.
