Hello how are you doing>?
Hello, I'm okay. I'm interested in your S5, what condition is it in?
It is in perfect working condition and I'm asking 120 for it, how does this sound to you?
I don't quite want to pay that much for a used phone. I was thinking more like $90. 
I can do that for the phone, that seems fair will you be able to pick it up and pay cash today?
Sounds fair enough. I can swing by after work and pick it up. 
Great!
Thanks for negotiating.