I've only been in contact with a 9420 once and that was a very long time ago, but none of the controls were overly sensitive and I doubt that was by default. A jumping control is a sign of a defective pot or encoder, not a design feature. It's a common problem with hardware that is 20+yrs old, and in general easy to fix.
I would not sell it if I were you. The 9420 is actually not a bad scope if you are aware of its limitations. If I remember right it has quite sophisticated measurement functions, more than what you'll find on a new beginner DSO.
You can still learn a lot from it, including how to handle a digital scope properly.