What do you think a fair price would be on the iPhone 6, 64GB, mint condition with no scratches or dings and with the original box, new (unused) headphones and charging cable?
Goto Swappa.com and see what other sellers are listed for. You can check at the bottom of the site to see that last 5 sold to get an idea.
With that said if you want it sold relatively fast than $620-$640. For the right buyer to come along I'd say around $680. Good luck. Try craigslist for a fast cash deal.
I would say around $650 range is probably a good starting point. I purchased a 'new' Verizon 64GB iphone 6 off of Swappa with no box or accessories for $600.