This is false.
This was fact-checked by the AP. They prove that the claim of Mother Teresa selling babies is being "conflated" with the case of an Indian woman named Anima Indwar, who is a member of the Missionaries of Charity: the same order Mother Teresa was part of.
In 2018, 21 years after Mother Teresa's death, Indwar sold a baby from her order's shelter to a couple for 120,000 rupees. Three other investigations of Indwar selling other children are ongoing. Mother Teresa played no part in this.
Evidence: https://apnews.com/article/fact-check-mother-teresa-charity-205334220046