What would you check before paying for a secondhand phone?

A used phone can look spotless in photos while hiding problems that only appear during an ordinary day. Battery life, charging, cameras, calls, and account locks all matter more than a shiny case. Meeting a seller can feel rushed, especially when they say someone else is ready to buy. What belongs on a short checklist that a normal buyer could actually use? It would be helpful to discuss the checks themselves rather than particular listings. There should be enough time to test the device and understand the return arrangement before money changes hands.