Activation Lock and account sign-out come first
Start here, because a locked phone can only be released by the previous owner. Apple's own advice is not to take a used iPhone until it has been erased and removed from the seller's Apple Account. If you see a passcode screen or someone's Home Screen, it hasn't been erased. If setup asks for the previous owner's Apple Account, it's still linked to them. A properly prepared iPhone shows the Hello screen. Android works in a similar way with Factory Reset Protection: if setup asks for a Google account that was on the phone before, the seller hasn't removed it. Ask the seller to sign out and reset in front of you.
Match the IMEI everywhere
Dial *#06# on the phone to show its IMEI. Compare that number with the one in Settings (on iPhone, Settings → General → About; on Android, usually under About phone) and with the label on the box if the seller has one. They should all match. A mismatch between the box and the phone isn't automatically proof of anything bad, since boxes do get mixed up, but it's a reason to ask questions and be more careful about ownership. While you're in the About screen, check the model name and storage too, so you know you're getting the exact version you're paying for.
Battery health and signs of swelling
On iPhone, go to Settings → Battery → Battery Health & Charging (shown as Battery Health on some iOS versions) to see the Maximum Capacity. On iPhone 15 models and later, the cycle count is also available. Android varies by brand. Pixel 8a and later show battery health under Settings → Battery → Battery health, while Samsung Galaxy phones can run a battery check through the diagnostics in the Samsung Members app. Other brands may not show it at all. Whatever the phone, look along the edges for a screen or back panel that seems lifted, which can mean a swollen battery. Lower battery health isn't a deal breaker, but the price should reflect it.
Screen and touch
Turn the brightness up fully and open a plain white page, then a dark one. On the white screen, look for yellowish patches, tints or dark spots. On the dark screen, look for faint ghost images from burn-in and any bright or dead pixels. Check for lines running across the display. To test touch, press and hold an app icon and slowly drag it to every corner and edge of the screen. If the icon drops or jumps, part of the touch layer may be faulty. Also look closely for cracks hidden under a screen protector, and ask the seller to remove it if you can't see the glass clearly.
Cameras, speakers and microphones
Open the camera app and take a photo with every rear lens, switching between zoom levels, then take a selfie with the front camera. Tap to focus on something close and something far to check autofocus, and try the flash. Record a short video while talking, then play it back, which tests the microphone and the speaker together. Call your own phone number from the handset to check the earpiece and the call microphone. Play some music at full volume and listen for crackling or distortion. Blurry spots in every photo, a lens that won't focus or muffled sound are all worth asking about before you agree on a price.
Charging, buttons and connectivity
Plug in a charger and make sure the phone starts charging straight away, without the cable needing to be held at an angle. If the model supports wireless charging and the seller has a charging pad, try that too. Press every button, including power, volume and the mute switch or Action button. If the phone has a SIM tray, put your own SIM card in to check signal, calls and mobile data, which also shows whether the phone accepts your network. Some phones sold through telco packages can be locked to one network, so test rather than assume. Finally, connect to Wi-Fi and pair a Bluetooth device such as earphones.
Face ID and fingerprint
Don't just trust that biometrics work because the seller says so. After the phone is reset, or with the seller's permission, set up Face ID or add your own fingerprint. On iPhone, a message saying Face ID is not available, or a setup that never completes, points to a hardware problem that may not be simple to repair. On Android, try the fingerprint sensor several times from different angles. Test the proximity sensor too by making a call and holding the phone to your ear: the screen should switch off. Biometric faults are easy to miss in a quick look, and you'll notice them every time you unlock the phone.
Parts and service history, and water damage
On an iPhone running iOS 15.2 or later, go to Settings → General → About. If the phone has been repaired, a Parts and Service History section appears there, listing parts such as the battery, display or camera with labels like Genuine or Unknown. Android has no single equivalent, so ask the seller directly about past repairs and receipts. For water damage, many iPhones with a SIM tray have a liquid contact indicator inside the tray slot, which turns red after contact with liquid. On any phone, look for corrosion in the charging port, fog inside the camera lenses and speakers that sound muffled.
Ask about warranty before you pay
Ask whether the phone comes with any warranty, how long it lasts and exactly what it covers, because terms differ between sellers and between individual units. Get it in writing, together with a receipt that shows the model and IMEI. A private seller usually can't offer a warranty at all, which makes your own checks even more important. ReDevice also sells pre-owned devices. Stock changes often and warranty terms depend on the specific unit, so ask on WhatsApp about the device you're interested in and what comes with it. Whoever you buy from, run through this checklist before you hand over any money.
