Q1: A bad idea because it is already broken-in in a way that means it is irreparably shapes for the previous owners body?
It is possible to reshape a leather saddle by first soaking it in water, resetting the shape then breaking the saddle in. I have seen it referred to as the "Blocking Technique" and have also seen warnings that it is possible to overstretch and even break the leather (although I am personally doubtful) so caution is needed.
You could use water to reset the shape, treat the saddle with proofide or minks oil. Let it dry then break it in the old fashion way (lots of miles)
Or you could use water to reset the shape, treat the saddle with proofide or minks oil, then break it in with a short ride (10 min) while wet to create a new shape. Then use successively longer rides to fully break in. See the tutorials below for more detail.
Disclaimer - you could also wreck the saddle if you do things like crank up the tension it while wet, or ride or a long time while soaking wet. If you have never worked with leather before, be prepared for the possibility of failure.
- Re-shaping Deformed Leather Saddles
Water Break-in Tutorials
- Breaking in a Leather Saddle - Rando Richard
- Breaking In a Leather Saddle - LON'S PACTOUR
Q2: A good idea because the leather is more supple and more easily bent and shaped to the new riders body?
The leather actually gets stretched to match the rider that breaks it in, so it is not more supple to newer riders. Ideally, for the most capacity to adapt to your body, you would want to start from an un-stretched (beyond factory stretching) saddle. That doesn't mean a used one can not adapt your body, but that your best shot is with a new saddle. Assuming your physical structure is not radically different, this point is likely more academic than real world.
Q3: An equally good idea because the leather will simply reshape over time?
Neither - as stated in my answer to Q2, the capacity for the leather to adapt to your individual shape will likely be less. Whether or not this will matter with the saddle you purchase will depend on a number of factors from your personal physical bone structure, and preferred riding position, to the condition of the used saddle.