Your logic is impeccable, but the devil is in the details and the conditions you establish. Even a conscientious seller may not realize how much damaged and degraded a rope might be - the only way I know to establish the strength of a used rope involves destructive testing. You also have another problem. You may designate a rope for rappel only, but situations will arise where you will be tempted or impelled to use it for leading or belaying. You might get away with that for a while (and you likely will), but sooner or later.....

The only rope worth loading onto your pack is one that is capable of full service. Once it is no longer serviceable for leading, only non climbing, non critical applications are appropriate.

I have to chuckle at my resolutely conservative philosophy here, especially when you realize that the first rope I ever used (incorrectly, I might add) was a 5/8" manila line that I later learned had come from a spool condemned by the telephone company. Since then I have witnessed entirely too many bad situations involving junk ropes. Cut corners and save money somewhere else.