If I'm missing how to do this, I apologize. Can STOCK have a favorites/heart feature to save for future purchases?
Not that I am aware of, you can certainly save preview to one of your libraries for the same purpose.
Then what the heck is the "heart" button for?!?! I favorited several photos and they are no where to be found
That "heart" button is to save previews - relatively new item we implemented.
Where can I find the saved previews?
You can download or save them to a library.