I've always sold my "older" phone. Usually on eBay or Swappa and I get pretty good for them. I got $685 for my Space Gray 6S 128GB in mint condition. However I've come to the conclusion that since I upgrade literally every year, and sometimes it's a hassle to sell my phone, I will be doing the Next Every Year with AT&T. That way I can upgrade every year and just trade-in my previous phone.
Yeah, I have sold some previous ones myself as well as give them to family members. I went with AT&T Next this year too. It's a perfect plan for me and seems pretty fair. Being able to trade in after 12 months will be much easier than finding a buyer and dealing with shipping it.