Yeah that's why I said I wanted to maybe meet at the Apple Store, they could see from thereI woudnt buy the newest device from a marketplace.
I bought my iPhone 6 for 220 euro's but I didnt mind that.
Make sure account is logged off, camera is working, bluetooth, internet when inserting sim, every side of the screen catches your finger movement by sliding.... etc etc
And check your goverments police website to be sure if its stolen or not. So that you wont get any problems.
Think about it this way...why would he trade if your getting the better deal? A bad excus would be "i got no time or money to repair it"
And it somewhat doesn't make sense, but if the device fully works then