The first thing to do is check the ESN serial devices are 'clean'. When an ESN or IMEI is not clean, it means that the iPhone has been reported stolen.
How to check is to go to Settings - General - About - IMEI. Then the iPhone that you want to buy, go to this link and enter the IMEI code to check the serial ESN.
The next important thing when buying an iPhone is to check your iCloud account. Ask the seller to sign out of iCloud, then asked again to remove iCloud account.
If he is not willing or failed to enter into iCloud, you should find another seller.
Severe or scratch the screen when turned already underlined is a sign that iPhone is not worth buying. Usually, if the screen is damaged, the other components are not prime.
Apart from the views, try also the sensitivity of the screen. If it is not responsive, this is also a reason not to buy it.
You can also test your iPhone which will be purchased by mecoba listen to music using earphones. If the sound is not clear, it is likely the sound from the speakers is not optimal when used for phone calls.