When looking for a car or bike, I always try to get a look at the log book or previous invoices to get the previous owners details and if possible give them a ring. Buying a car for the wife, I called the previous owner who told me the history and the real reason she part exed it for a different car. She needed more space. I recently sold my Suzuki Bandit 1200 to a local dealer. The bike has somehow gained a full service history which it did not gain in my ownership. if a prospective new owner called me and asked for details I would tell him the truth, that the bike was serviced by me from 1500 miles when I bought it to 15000 miles when I sold it and it never saw a dealer for servicing in that time.
I've always been tempted to call the previous owner of my bike and ask why he sold such a high spec bike so cheap ;D
Forums can also be a useful way of contacting previous owners or finding out the history of a bike.
I part exed my CBF1000 when I bought the GTR and not long afterwards I had an e-mail from a chap who had seen it at the dealers and asked if it was mine.
They had joined the CBF1000 forum, as a lot of people do these days, to find out what the CBF1000 was like as a bike and had read a couple of posts from me saying that I was/had part exed the bike. I was more than happy to respond and assured them that although it was a high mileage bike (32,000) it had been serviced and looked after etc etc. They also benefited as I had a few extra bits and pieces in the garage that I hadn't passed on to the dealer.
Steve