A used phone can be blacklisted, locked to one carrier, or a year from its last security patch. Check all nine before you pay. Number nine is the one people skip.
Dial *#06# to show the IMEI, then run it through CTIA's free Stolen Phone Checker. Five checks a day, US only. Red means the phone was reported lost or stolen.
Play Store, tap your photo, Settings, About. It shows Play Protect certification. Uncertified phones are not licensed for Google apps and may never get updates.
Settings, About phone, Android version shows the last security patch. Pixel 8 and Galaxy S24 get seven years. Older models can be nearly out of runway.
Pixel 8a and newer: Settings, Battery, Battery health. Reduced means under 80% capacity. On Galaxy, open Samsung Members, Support, Phone diagnostics.
Open a plain white image, then a black one, at full brightness. Dead pixels, burn-in and blotches only show on a flat colour, never on a busy home screen.
Samsung Members Diagnostics checks more than 24 items: cameras, speakers, sensors, touch and SIM. Two minutes there beats trusting a stranger's description.
Plug a cable in and move it around. Charging that cuts out, or a port that feels loose, is a repair bill. Test wireless charging on the same phone too.
A carrier-locked phone only accepts SIMs from the network that sold it. Bring your own SIM, make a call and load a web page before any money moves.
Factory Reset Protection ties the phone to the seller's Google account. They must remove that account and reset it while you watch, or you own a locked brick.
Nine checks take five minutes. Do them with a seller who offers a return window and a bad phone costs you an afternoon, not a few hundred dollars.