Retribution depends not only on the person wronged and the aggressor. You've fallen into the usual timeworn fallacy, the belief that a society with no government has no laws and no order; that it's all atomic 1 to 1 interactions.
This is 180 degrees from reality.
Did you watch the video of the intersection where the lights were turned off? What did you see? Mad races to get through the intersection, or an orderly pattern of traffic? In the Real World, people negotiate order when left free to do so. In the Real World, that voluntary order is more efficient than that imposed from without; traffic flow increased; safety increased.
Where do you see mad races to get through an intersection? Where there are traffic lights, especially with traffic cameras. ( I witnessed many such instances in Los Angeles, including a few accidents. )
Back to the problem: why would retribution be proportional? Why not kill everyone for the slightest aggression?
Answer: because no one would want to live in such a society. You might be happy to know that no one would dare lift even a piece of penny candy from your store, but you'd be unhappy to know that, if you are suspected of even a slight act of aggression, you'd be killed. On balance, it is better to live in a society where punishment is proportionate to the crime.
An AnCap society, over time, would evolve a body of customary law which would tend toward proportionate response; it would tend to protect against arbitrary responses, such as this hypothetical "murder and sell organs" response to "not paying for a sandwich" scenario.
In an AnCap society, you don't become a god with power of life and death merely because someone has wronged you. That's statist thinking, the thinking of a King or a politician. In an AnCap society, the accuser is still just a person, like everybody else. The accused person is still a person, not a subhuman with no rights.
What does anarchy mean? It means no ruler. You don't magically become a ruler by claiming that somebody aggressed against you. There are no rulers. There are no privileged people. Nobody is more equal than others. You don't get to kill people and sell their organs just because they didn't pay for a sandwich. Your response has to be proportionate, or you are guilty of an act of aggression.