Trading through Apple tends to be the "Safest" method - they're not likely to scam you, as long as you're honest about the condition of the phone.
You can get more money by selling the phone directly - but there's a hassle-factor, and the possibility of being scammed. I live in a small-ish town so the other possibilities are mostly online, where scams are more common. Meeting someone locally & selling for cash reduces that possibility, but that means increased hassle - and the possibility of being robbed, etc.
Personally, I just use Apple & take what I can get.