Apple.com or Apple store.
Sure, you’ll be paying full price, but then you know you won’t be cheated. There are so many scammers out there that it’s just ridiculous.
I personally fell for one. Seller claimed it’s a new iPhone, but when I received it, the box is fake and the iPhone itself was already registered under someone else’s name and its warranty has expired when I check it with Apple. And this is from a legit e-commerce, not a Craigslist or something like that. Good thing they had proper return policy so I got my money back.