We've detailed the process for returning your Galaxy Note 7 if you had purchased through a carrier or retailer, but the phone had been out long enough that there were plenty of used and second-hand phones floating around out there as well. Thankfully you won't be left out in the cold if you bought a used Note 7: Samsung has a process for you to send in the phone and get a replacement.
Here's the full statement from Samsung on the situation:
Any customer who bought a Note7 [used] should contact us directly at 1-844-365-6197 to exchange their device. We are working as quickly as possible to make the service experience for our customers as quick and smooth as possible and we are grateful to our Note7 customers for their patience during this time.
So rather than trying to track down the original seller, or go return the phone to a carrier that you didn't buy it directly from, just go straight to Samsung by calling 1-844-365-6197. The policy stands now as it did from the first hints of a recall back in September: you can turn in your Note 7 to Samsung and get it replaced with a Galaxy S7 or Galaxy S7 edge. Samsung will of course arrange for the return of your Note.
This is the right move from Samsung, for sure, and will help foster a little good will amongst those who bought secondhand phones with no warranty or receipt of sale.