1. Meet the seller in person in a public place.
2. Inspect the device for damages.
3. Have the seller power on the device and then have him verify that the iCloud Activation Lock has been disabled (To do this, the seller has to disable the toggle next to Find My iPhone and then enter his iCloud passcode to confirm)
4. Confirm that the seller is signed out of iCloud, the iTunes & App Store, iMessage, FaceTime all mail accounts, reminders, notes, calendars, etcetera.
5. Have the seller go to Settings-General-Reset and then select the erase all contents and settings option in your presence.
Afterwards, and if the price is to your satisfaction, purchase the device, go home and set it up. These are my recommendations.