Any reason to believe they'd actually do this? Apple are one of the few companies that seem to genuinely value customer privacy[1]; They make their money selling your products, not selling your personal information after all. If this were a Google or Facebook product on the other hand...

[1] I could point to numerous examples of this (removing APIs used to track individual iOS devices, fighting the government on proposed encryption backdoors, etc), but I think the onus is on you to rather provide evidence to suggest this is something they would do :)

This is an ridiculous suggestion. Apple's products have had minimal data reporting and a focus on privacy. They even conceded awkward UI and bugs around messaging for the sake of keeping it secure. Their advertising project failed partially because of their unwillingness to track users. They famously took the US government to court and won in order to prevent them from being forced to unlock a phone, for fear of the precedent it would set.

What possible reason could you have to assume that they would sell health data, for which all of the same reasoning applies as well as it being more heavily regulated by the government?