Start with the model and storage
A phone or tablet listing should give the exact model and storage capacity. One model family can have versions with different network, SIM and connectivity support. If you need a specific feature, such as mobile data on a tablet, check it separately.
Judge software support by the manufacturer's current information. The latest available update and a promise of future support are not the same thing. An older device may be fine for simple tasks but not meet the requirements of an app you need.
Locks and battery
Before buying, find out whether the previous owner's accounts and activation locks have been removed. A perfect screen won't help if the device can't be activated after a reset. Don't treat an unclear account status as a minor cosmetic issue.
Remaining battery capacity is useful if it is stated, but it doesn't replace a working test. Check the charging port, cameras, microphone, speaker and biometric features if they matter to you. «Powers on» doesn't confirm that all of these work.
Assess the actual unit
Look for cracks in the screen, bends in the body and any known repairs. Choose from the photos of the specific lot, not from marketing images of the same model. Check whether the charger and cable are included, and who is selling it.