The woman was arrested in October during a police sting at a local KFC restaurant following a tip-off that the 20-year-old had offered her son for sale on the popular advertising website.
"The mother pleaded guilty to trafficking in persons, money laundering and contravening the Children's Act by putting her child up for sale," National Prosecuting Authority spokeswoman Natasha Ramkisson-Kara told AFP.
In a statement read out in court in the eastern coastal town of Pietermaritzburg, KwaZulu-Natal province, the woman said she tried to sell her baby after her boyfriend was found not to be the child's biological father, reported local news website News24.
"The money was in a bank bag which she took from her handbag and told me to count it, which I did."
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"She asked if I wanted to leave and I said yes. As I got up, I was approached by two female police officers and they said I was under arrest."
The woman is currently on bail and has to report to a police station twice weekly.
Officials said the baby was being cared for, but declined to give further details.