'Zinc brake wire core’ doesn't mean anything intelligible. Zinc is far too weak to be strands for a brake cable; as discussed this probably comes down to galvanized steel (for rust protection, the old way); but you never know with "everything's made in China" and so forth.
I don't see the point of buying galvanized cables to save a few pennies or even a buck.
I never buy something that's not in a reputable manufacturer's packaging on eBay, at least not anything that would be worth someone's while to counterfeit as a business model: always a judgment call there.
Amazon: I'm a never-Bezoser; I don't like the guy or his business methods. Always go to a supplier direct, preferably one that I'm reasonably certain doesn't sell on Amazon.
So: Jagwire from one of the online direct places usually. LBS if I really need it today.