> Phones are nice and self contained enough that looking at the outside generally tells you the condition of the inside.

It doesn't say anything about the condition of the (now commonly non-user-replaceable) batteries and the battery longevity. The device may not have any scratches or dents, but the owner might've used it primarily to watch videos and play high-intensity (graphics wise) games most of the time. Those actions make the battery a "needs to be replaced sooner" thing (or worse, make the phone a "needs to be replaced sooner" thing). Even checking the battery cycle count is inadequate because it doesn't say how quickly the drain happened during use (that also impacts battery longevity).