Well , the last 2 outboards I've sold, a 15 and a 25 hp, I offered to take them out fishing and they could drive etc... needless to say they were both sold, cash in hand once we reached the dock at home.
So ask if you can take it out for a rip or a little fishing first, if the guy allows it than he believes the motor is in good shape and you will buy it, turn it of /on a few times, go full out, troll slow ect.. and just see how it's running.
To check the oil you just have to take the screw out for 1 second, you can tell by the color of the oil if water was in it, but even if there was usually a new O ring fixes the problem.