ALT-1 Buying used Motorcycles
Two words; "adult owned".
If purchasing from a private seller and the seller is under the age of 25, run don't walk, to your second choice. Check that. If there is a male occupant of the household under the age of 25 move along. Ever been under the age of 25 before? Ever "borrow" the old man's wheels before? Speed kills. So does neglect. A typical 45 yr. old spends more time waxing and maintaining than riding. Ever see a 45 yr. old doing' a "wheelie" or an "endo"? Me neither.
If purchasing from a dealer ask to see the service record. Most dealers only take trades on what they've previously sold as new. owners are more likely to have vehicles serviced by dealers. If no service record is available, request a carfax vehicle history report. Yes, Carfax covers bikes too. While you won't get the age of the owner from carfax you may get some clues and other useful information. Was it wrecked, stolen or impounded? Ever heard the expression "ride it like you stole it"? If all else fails, ask the dealer who the previous owner was. They almost always have a photo copy of a drivers license in the sales transaction file, you're only interested in the year of birth, shouldn't be a deal killer (if they were over 25, right?).
Along with checking blue book values I would also check for any safety and mechanical recalls. It's free and should take less than 5 minutes (a little longer to print out so you can check with the dealer if said recalls have been completed) http://www-odi.nhtsa.dot.gov/cars/problems/recalls/recallsearch.cfm. Don't be put off by the fact that you found recalls for the bike of your dreams, in fact I'd worry more about finding zero recalls. Every responsible manufacturer wants to keep owners happy AND safe (can you say repeat sales) thus the majority of recalls are Voluntary (initiated by the manufacturer and not a governmental agency). So there are usually more than a few for each vehicle. Most of the time they are just going to be visual inspections by trained eyes. The important thing to look for here is whether or not recalls were completed and by whom.
After the above steps are followed, "kick the tires". Check for scrapes on mirrors and turn signals (items that would make road contact first in a spill). Better yet, have a mechanic "put an eyeball on it". Then ride it like you stole it, just be sure to keep the shiny side up. And wear a "lid" that's D.O.T. approved (No Novelty helmets).