Once again, I'm going with James 100% on this one.
I once completed what you might consider to be a "pre-purchase inspection" for a friend of mine. He had been eyeing a Kawasaki Ninja 650 that he'd found online, and so I met him at a local parking lot where the owner rode up on his bike to perform his sales pitch. It didn't take long for me to pull my friend aside and tell him he was wasting his time--this bike was junk.
The sad thing is that the bike was less than two years old, but its condition was a disgrace. The tires were in poor condition, the chain was rusted so badly that the formation of several frozen links could be seen, the cables were rusted (with the clutch cable partially eaten through), there were scuff marks everywhere from multiple crashes, and the oil had the consistency of chocolate syrup. This bike was less than two years old, had 18,000 miles on it, and probably wouldn't have made it another year.
The 21-year old rider really didn't think there was anything wrong with the bike he'd purchased new just 18 months before, so while my friend pulled away in his car, I had a brief discussion with the seller about what he probably needed to do better next time.
On the other hand, I purchased a 2003 Ninja 250 for my wife last year that was personally owned by a dealer mechanic. People often use the expression "it looked like it just came out of the showroom", but in this case, it really did. I could find NOTHING wrong with it, and as a result, I wasn't at all scared off by the 38,000 miles on the odometer.
Finally, one of my street bikes is a 25-year old Ninja ZX600A...with 91,000 original miles (though I admit it's consuming about as much oil at this point as gasoline, and will finally require rebuilding this summer).
My advice is to have a reputable mechanic check the bike over if you're not sure what to look for. In any event, mileage means nothing if the bike wasn't abused.
Good luck, and ride safe.
Buyer of lots of old bikes