iPhone 7 warranty - When does it begin?

The warranty begins (or in your case began) on the initial date of retail sale by Apple or an Apple Authorized seller. There are no eBay authorized sellers, so any and every iPhone sold on eBay is a used device, whether activated by anyone or still shrink wrapped in the box.

I'm sorry, but the seller is completely wrong. I don't know who you purchased AppleCare+ from but Apple warranty absolutely begins the day the phone was sold out of Apple's inventory. You visited Apple directly and they told you so. The seller cannot override Apple. Only Apple can make an exception so unless you have confirmed with Apple the warranty is valid, you still don't have Apple warranty.

The warranty begins when the phone is initially sold, in the case of your phone, that was 2016.

I don’t know what actually happened in your case, but the warranty clearly begins at purchase. See - Apple One (1) Year Limited Warranty (my bold emphasis):

“Apple Inc. of One Infinite Loop, Cupertino, California, U.S.A. 95014 (“Apple”) warrants the Apple-branded iPhone, iPad, iPod or Apple TV hardware product and the Apple-branded accessories contained in the original packaging (“Apple Product”) against defects in materials and workmanship when used normally in accordance with Apple's published guidelines for a period of ONE (1) YEAR from the date of original retail purchase by the end-user purchaser ("Warranty Period"). Apple’s published guidelines include but are not limited to information contained in technical specifications, user manuals and service communications.”

You’re welcome.

I hope for your sake this works out as you think. I have never heard of Apple ever granting a one year new warranty to a product which was originally sold from their inventory in 2016.