If someone gives you a gift

Do you feel that the gift is yours to do with however you please OR is it your obligation to keep it?

As an example from Reddit. A lady was giving all of her baby items and hand me downs etc to her sister in law. A crib, clothes, toys..because the sister in law has a younger child. She found out that as a "side hustle" that the sister in law has been selling all the items online.