Make sure the device isn't stolen
Check the serial number of the iPhone against the CheckMEND database. Not all stolen phones are registered on this database of course, but it's worth checking if you have any doubts. There's a small fee of £1.99 to check the database, so only do so once you're sure you want to buy the phone.
Make sure that the seller has turned off the activation lock on the phone and followed Apple's instructions for what to do before selling an iOS device. You can confirm that the activation lock has been disabled on a particular iOS device by using the Check Activation Lock Status page.
Only people with access to the Apple ID associated with the phone can disable the Find My iPhone feature, so this is extra proof against a stolen device. If the device has been reported as lost on Apple's Find my iPhone service, it will be listed as such on their activation lock status page.
Check for damage before buying it
Be wary of phones that have had repairs. They may have been repaired to a low standard, especially if undertaken by a have-a-go-Joe that does it in their spare time. Quality iPhone repairs are a very skilled process requiring expensive specialist tools, quality parts, and a lot of experience. If the phone has had a repair, ensure it was done professionally to avoid problems later.
Check the screen and outer casing for damage such as chips, scratches, dents etc. Also make sure that the screen is not lifting from the frame, this can be a sign of a previous poor quality repair and can lead to more issues later on such as dust getting into the unit.
Check that all of the buttons work. Make sure that the volume buttons, home button, sleep button and silent switch all function correctly and don't feel weird or sticky as this could be a sign of dirt trapped in the buttons.
Make sure that the touch screen and LCD work. The quickest and most effective way to check that the touch screen works is by pressing and holding one of the app icons on the home screen and moving it around the screen, making sure that you've moved it around every part of the screen. If the icon drops without you lifting your finger, you have a faulty digitizer which will need fixing. While doing this, make sure there are no dead pixels or glitches on the screen. You can open a blank page on Safari to make checking for dead pixels a bit easier.
Check all of the cameras. Make sure that both the front and rear cameras display an image in the Camera app and also that they aren't hazy and focus on things as expected. On iPhone 7 Plus models, check the dual rear cameras by zooming all the way in and all the way out and making sure it doesn't lose the image.
Open the compass app, this is normally found in the Utilities folder on the home screen. From here you can test the compass and gyroscope. Check that it works ok, and then swipe it to the left to access the spirit level and make sure that this also works.
Make a phone call. This tests that the phone can transmit mobile data ok and also the microphone and speakers. Be sure to test both the regular call volume and loud speaker during the call. During the call, also test the proximity sensor by covering the top half of the screen with your hand. If the screen goes blank, it's working fine.
You should also recieve a phone call to the device. This tests that the vibrator works correctly.
Check that the headphone jack works on models prior to iPhone 7 by listening to some music or sounds through headphones. If you have earphones or a headset with a built in microphone that's designed for use with iPhones, you can test that the TRRS contacts aren't damaged by making a quick call to somebody and asking if they can hear you ok.
Make sure that the Wi-Fi antennas are working fine. This can be done by opening Settings, enabling Wi-Fi and making sure that it sees any nearby networks.
Check that the Bluetooth connectivity works by opening up the Bluetooth menu in the Settings app and making sure it can see other devices. Try it with an other phone's Bluetooth enabled and set to visible if no devices are listed.
Make sure the storage capacity is what was advertised in Settings → General → About.
Check the battery's health by going to Settings → General → Usage. Ideally, also find out how long it takes to go from fully charged to completely flat. From this section in the settings app you can see the total amount of time the phone has been switched on and being used, and also how long it has been in standby mode with the “Usage time” and “standby time” sections. This can give you a good indication of how much the phone has been used, similar to a car's odometer.
Check that the dock connector works by plugging it into a computer, laptop or other device that uses the connector for both power and data, and ensure that the phone is recognised. Also make sure that the phone charges. So-long as the battery is also healthy, it should indicate that it's charging when plugged in. If it doesn't work at first, make sure that the cable you are using is definitely working too.
Look for any hints of water damage. iPhones have various LCI (Liquid Contact Indicator) spots, one of which is inside the headphone jack, the other one is inside the dock connector. There are two others inside the phone, but they're only visible if the phone is opened up. If they have been in contact with water, they will turn red. Otherwise, they'll be white.
I don't recommend buying a phone that's had any liquid damage as it's not worth the risk, even if it appears to be working fine currently. The liquid damage could be causing corrosion internally which will eventually damage things.
Check that the GPS works by opening the Maps app and making sure that the little blue dot shows your current location accurately. Bare in mind that it sometimes takes a few moments to determine your exact location. For better results, disable Wi-Fi while doing this step as the phone may be determining your location using the Wi-Fi network it's connected to.
Make sure there isn't any freezing or general weirdness during your testing that doesn't quite feel right. This could be the early signs of a failing phone or an issue with it's power regulation, heat, RAM or storage.
Other things to consider
If the phone is locked to a certain mobile phone network, make sure it's locked to the same one you're going to be using it with.
If you don't already have an appropriate charging cable for the phone and the seller isn't offering one, this is an extra expense to consider.
If you decide to buy the phone, as soon as you gain ownership, completely factory reset it. This can be done from Settings → General → Reset. This should work fine so-long as the previous owner has removed the activation lock on the device. Be sure to tie it to your Apple ID and consider the Mobile Safety tips I've written about previously to keep it safe.
For your convenience, I've put together a simplified checklist:Download a printable checklist (pdf)
Please print this checklist or fill it out digitally when you inspect an iPhone you're interested in buying. It's designed to help jog your memory on what you should be looking out for. Hopefully it helps to make sure you don't forget anything!
If you've found this guide useful in assisting your decision to buy an iPhone, or to avoid one that seemed fine at first glance, I'd love to hear about your experience! Feel free to get in touch.Written by James Allen, September 2016