jmherbenerThe subjective value theory does not say that there can be no objective way of determining which course of action is ethical better.
Subjective value refers only to the fact that each person’s valuation of alternatives in action exist in his mind. It is an intensive state of mind that has no extensive property. Without an extensive property there can be no objective measurement of valuation since an objective unit of valuation cannot be defined. This fact implies that the subjective valuations of different persons cannot be objectively compared. Therefore, when a social interaction benefits some and harms others, there is no objective method to determine whether society has more or less utility.
Subjective value, however, does not imply that there can be no grounds whatsoever for an objective ethical system. Just because one cannot objectively demonstrate that a social interaction that benefits some and harms other is better in the sense of generating great social utility doesn’t imply that there are no other grounds on which the social interaction could be objectively deemed better.
Murray Rothbard, in his book The Ethics of Liberty, famously grounded an objective ethics in the human nature and natural law. Here is an article by Rothbard on the topic:
https://mises.org/library/justice-and-property-rights-failure-utilitarianism
Also, even though one cannot objectively determine the social utility of a social interaction, a person can make a judgment that it seems likely that the gain to one group outweighs the loss to another group. Such judgments are the basis for economic history as opposed to economic theory. Here is an article by Joe Salerno explaining how the economist uses his own judgments in doing economic history:
https://mises.org/library/how-do-economic-history
As you say, even if one judges the outcome of a social interaction to be socially beneficial, it doesn’t logically follow that violating property rights to bring about the social interaction is ethical.