I always say this, you're going to buy it. They're going to sell it. What's this thing worth to you? Honestly you're going to sell it too.... So say for instance, you're looking at a 2-3 year old bike that starts at 11k out the door but you want to buy it for 50% of its original value, $5500. Then you're inevitably saying that in 2-3 years you'd be willing to pay $2750 a 50% haircut. I don't know about you but I wouldn't buy into something that holds its value so badly. If you want a pristine bike then you're going to pay a lot more. Obviously there's a real reason why someone would sell their bike for such a large hit, motors bad, laid down, needs repair.... KBB and NADA are just guidelines to get you a good idea of what an item is worth. They compile statistics about final values in vehicle sales. To be honest though, they don't take into consideration all of the bikes that need repair when sold so they're unreliable and almost always too low.
Just my .02