The National Prosecuting Authority (NPA) says that the mother faced charges including human trafficking and money laundering.
The 19-month-old boy is now in the care of social workers.
The mother, who appeared in court in Pietermaritzburg, was arrested after a police agent posed as a buyer.
She will now remain under house arrest until her next court appearance early next year.
NPA spokeswoman Natasha Ramkisson-Kara said more investigations would be carried out into the mother's personal circumstances.
"She will appear on 29 February but before that they will compile a pre-sentencing report which will be done by social services," she said
"Social services will do a background check of the mother and study her upbringing and then once they complete that study they will then hand over that report to the magistrate in court," she added.
Gumtree has been co-operating with investigators, the police said.
A 20-year-old woman in South Africa has pleaded guilty to trying sell her child for 5,000 rand ($380; Â£250) on advertising website Gumtree.