Chalifin for a MInor - Gittin 65

Rava said: There are three categories of a minor: If he would be given a rock, he would discard it, but if given a walnut, he would accept it, he can make an acquisition on things, but he may not acquire for others.

Tosfos asks that it seems from a Gemora in Bava Basra that a minor does not have the ability to acquire anything!?

Tosfos answers: The Gemora there could be referring to a minor who has not yet reached this stage of understanding.

Alternatively, they answer that the Gemora there is referring only to a kinyan chalifin (acquiring something through an exchange with a kerchief or other object), where a minor has a more difficult time grasping the mechanics of the kinyan.

The Ra”n explains that the Rabbis instituted for a minor only those kinyanim where the object is raised or pulled by the minor; however, other types of kinyanim, where the concepts are difficult to grasp, are not effective for the minor.

The Rashb”a adds that since we rule that the object being used for the chalifin must be owned by the one making the acquisition, it emerges that the minor must convey the object to the seller. He does not have enough knowledge to accomplish that and therefore the kinyan is not effective.