I generally try to avoid electronics used, unless it is cheap at $20 or something.
I know this may make me sound like an oldie, but honestly they do not make electronics like they used to. All the stuff I bought as a kid; walkman, stereo, gameboy, camera etc., is all working in good shape today. Nowadays a lot of stuff even from reputable companies seems to fall apart well before it should. I guess it is because electronics are more complex? I don't know.
Oh and another situation where you can buy a used cellphone, is if it is still under some sort of warranty, and the guy just needs mon mon.