Only buy if you are 110% positive that it hasn't been stolen. The seller should be able to tell you a story about why he had the bike and why he is now selling it, including showing you receipts and explaining how he got scrapes on it. Your own experience will also tell you whether he's a cyclist or a scrote. If he's "selling it for a friend, don't really know much about it mate" then assume the worst and walk away.
Buying stolen bikes creates a demand for them which leads to...more stolen bikes.