well it is a good way to judge the condition of the engine, the lower the comp reading the more worn the engine is, but then again, they could replace the rings and get the comp rating back up before selling...
so take it with a grain of salt, do a check to see what condition the motor at teh time of purchase .. rather than trying to use it as a guide, since there is no way to know that a 30,000 km car hasn't been absolutely thrashed without changing oil, fluids etc. motor could be just as worn as a 80,000 km engine that has been looked after a little.