Forget the Nikon, switch to Canon. So…, which Nikon lenses did you say you have for sale?
But seriously, not setting the lens to the smallest aperture before mounting it will not damage anything. At worst the camera will refuse to function until you set the lens to the smallest aperture ("FEE" message). The manual also says to turn off the camera before changing lenses, I don't think I've ever done that and I have yet to damage an electronic Nikon (since 1990).
I believe the failure of your camera is age related, something inside is drawing more power than it should. Did you notice a shortened battery life by any chance?
Cheers
Steffen.