First things first, my sincere apologies if this isn't the right place to ask for advice.
I have an iPhone 6s 16GB Silver Unlocked (GSM) in great, working condition that I'm looking to sell or trade. I've looked at pre-owned prices on Craigslist and eBay so far. They seem to be all over the place depending on condition and seller. Low side seems to be around $150 and high side seems to be around $180. More if the are any accessories are included with it.
If I trade it in at an AT&T store it's $80 instant credit and Gamestop is offering $72 in credit. What would be an estimated fair price for it and where should I list it to sell or trade, preferably local?