Quick reference list:
1) Verify unlock status (activation/carrier lock)
2) Touch ID (Does it work)
3) ICloud lock (Do you have the option to setup as new phone?)
4) Check front and rear facing cameras
5) Check microphone and speaker function
6) Check battery health
7) Check IMEI# on imeipro.info
These simple steps help save you from buying faulty or stolen iPhones, and potentially hundreds of dollars.
For iPhone X users most of the same is true with the exception of the FaceID which you will need to have the seller remove before you purchase.
Never buy a phone that is:
1) Password locked
2) iCloud locked
4) not charged at time a sale
*****Never take a seller at their word unless you know them.*****