The Torah states that a father may sell his daughter into servitude (Exodus 21:7). The Mekhilta asks the next logical question: if a father can sell his daughter, can a daughter sell herself? The verse answers emphatically. He sells her, but she does not sell herself.
Once again, the Mekhilta walks through the logical argument that someone might construct to reach the opposite conclusion. Consider the case of a son. A father is not permitted to sell his son, yet a son is permitted to sell himself into servitude if he becomes impoverished. The son's right to self-sale exists despite the father having no power to sell him. Now consider the daughter. Her father is permitted to sell her. If a son, whom his father cannot sell, can sell himself, then surely a daughter, whom her father can sell, should also be able to sell herself. The logic appears unassailable.
The Mekhilta blocks this inference with a crucial distinction. A son who sells himself has a specific characteristic: he can also be sold by the court for theft. His sellability as an adult, both voluntary and compulsory, is part of a unified legal framework. A daughter, by contrast, is never sold by the court for theft. Her father's ability to sell her is a unique and limited power that does not extend to self-sale.
The distinction matters because it reveals the nature of the father's power over his daughter's servitude. It is not a general principle of female sellability. It is a narrow, specific authority granted to the father alone, under specific conditions, that cannot be transferred to the daughter herself. The power to sell belongs to the father. It never becomes hers to exercise.