Terryallan explained it well. It's not necessarily that the rig is better. It's just that nobody has ever used it. I wish I had the money to buy new for that reason. Our friends who buy new have just as many different problems with their MH as we have with our old one. It's just that theirs is still under warranty.
When buying used, we like to buy from a private party. That way, we get a feel for the owners and how they've used and maintained the rig (or not!). One time I knew more about this guy's MH than he did! Not someone I want to buy from. And dealers don't know squat about the used rigs.