Tell me why you are so sure the phone was not stolen long before the OP acquired it. I believe that the OP feels it is not stolen.
OP Post:
"Hello. Basically, I purchased a used iphone 6 plus and it worked for about 2 weeks and then was suddenly reported stolen by the seller without the possible question of failed payment, what is the sellers motivation? He is being unresponsive - again, why ? and it honestly is not worth civil court. So, my question is, if i was later to "have a problem" with my phone and it would need to be replaced, would apple check the esn number of it? Would they give me a phone with a bad esn? Or would they replace it with a phone with a good esn? "
His unresponsive friend may not be the person that reported it stolen. The original owner may have done this.
This is only my thought process. it doesn't make it real but it is how I look at the post. We, or he is not sure how long it has been in the wild.