"The computer could be slow, infected with a virus, or have damaged hardware and you might not realize this until it's too late. Buying a used laptop or computer is a gamble when it comes to computer safety."
---you tell the owner you're a serious buyer
---if you can test it presently with whatever software you're going to use for an hour
---play music, run videos,
----test all ports,
----battery,
---you tell them to leave it running for a couple of hours
----if they're bothered dont even bother....
---run it through like you would do for a car