I live near an Apple Store and I've always offered to meet at the store, or in the mall of said store. I've had s person ask if they could go in the stir (with me) and confirm serials and all that with an apple employee. I was happy to oblige and so was the Apple employee.
The more transparent you are in your posting on where you will meet, I find, the less BS offers you get, too.
I haven't sold s phone since the 5s, so it's been about two years, but I can't imagine much has changed. I do find that after the new phone release the market becomes so saturated now that I stand to make something like $50 over just trading the phone in to Apple or a carrier.that said apparently trade values have come down this year so that may have changed some.