I think this has an extremely easy answer… Since Sentinels will be new to the standard library, give them a new standard library exception to raise… my personal suggestion would be raising something like ‘SentinelComparisonError’ which is either a subclass of ValueError, or a subclass of SentinelError which could potentially be a subclass of ValueError.
Since existing code using sentinels will likely (in my experience) be using existing custom exceptions, and code adopting sentinels once they are in the standard library will have no established exception yet, it seems most straightforward to define a new exception to go with the new functionality. The bikesheding here feels like it should be only be if “ValueError” is appropriate as a parent class due to potential interaction with existing code.