Buying an iPhone 6s from a small unauthorised seller. When does the warranty with apple start?
Since it was new in box and sealed, then the warranty starts as soon as you activate it.

The phone's serial number is tied to your Apple ID (which you'll need to activate it), and they'll know that it's a serial number that has never been used before.

The only issue you might run into is the country of origin.  Apple makes different models for different regions.  So if you live in the U.S., for example, and you bought it online, you might have difficulty with a warranty claim if it's a European model.  But other than that, you're good.