I like to interview the seller a good bit about his ownership of the bike. If the owner hasn't had the bike long and/or doesn't have any history of the bike, then my valuation of the bike and interest in purchasing drops rapidly.
If the owner has kept the bike up the value is higher than if not. This is easy to determine very quickly; consider the owner's attitude and looking at other bikes in garage, looking at maintenance records, and looking at the bike for sale should all give a consistently good vibe. A bike that won't start or has obvious signs of neglect will get a bottom valuation from me (and I'm very unlikely to be interested in purchasing such a bike).
If the owner has kept the bike up the value is higher than if not. This is easy to determine very quickly; consider the owner's attitude and looking at other bikes in garage, looking at maintenance records, and looking at the bike for sale should all give a consistently good vibe. A bike that won't start or has obvious signs of neglect will get a bottom valuation from me (and I'm very unlikely to be interested in purchasing such a bike).