My general rule of thumb on bike pricing. If it's over 10k miles old and relatively new, the max price I'll pay is $5k. If it's Italian, add on another $2k. It it's a superbike... well I dunno.
Under 10k miles on it, and knock a grand off per year since it was new.
Any crash damage, just stay away.
But that's a very rough guideline. Great thing about bikes is people are always scraping their knee or having babies and then trying to get rid of them.