generally, falls major enough to retire a rope after, are extremely rare. they say to retire the rope after a factor 2 fall. this would be a fall that is twice the distance of the amount of rope between the belay and the climber. for instance, if a climber climbs up from a belay 15 feet, puts no gear in, and falls, he will fall 15 feet back down to the belay and another 15 feet past it before the rope stops him. a 30' fall on 15' of rope. 30 divided by 15= factor 2. on the other hand, if a climber climbs 50', puts in a piece of gear, climbs another 50' and falls, not counting for rope stretch, he would fall 100' on 100' of rope. a factor 1 fall. yeah, he fell 100', but it was on a much higher percentage of the line, and thus a higher percentage of stretch/shock absorption. this 100' factor one fall would not be cause to retire the rope. most falls are even much less than a factor one. a 40' fall on 100' of rope would only be factor 0.4.
retired ropes are often retired from lead climbing, but still used (by the origional owner) for toproping since the forces involved would be much lower. these things were designed for very heavy shock loading, and should be able to support bodyweight easily even after it's retired from lead climbing. of course, you would want to check the rope for interior damage by carefully running it through your hands, feeling for soft or swollen spots in the core as these are signs of interior damage and looking at the condition of the cover to get an idea of how much it's been used.
i'm fairly certain that there has never been a climbing accident where the rope broke. ropes have gotten cut before but never just snapped due to weakness. they say the human body would not survive a load strong enough to break the line.
you are taking a chance when you buy a used rope, especially if you don't know you can trust the seller. you would probably be ok if you talk to the person to find out it's history and then inspect it before use, and of course you probably shouldn't use it for lead climbing. but if you are confident about inspecting a rope, it's probably because you've been through several ropes yourself and seen what worn and damaged rope looks like, in which case you don't need to buy a used rope because you have a few in the closet already.