Blue often results from overheating due to a lean mix, but it's not on the bends or near the engine, so I'll say it's not that. Because it's on the straight bits and strangely speckled, I'll punt it's excessive carbon deposits heating up. Maybe it was too often idling or lugging, ask the owner about its history (like maybe he delivered mail at some stage)
Hopefully it's an after market exhaust but it looks original I think.
Stick your finger deep up the exhaust, you should have a small amount of black on your finger (like you probably will from your car for comparison) it should be dry, not oily.
I don't think it would be from running rich because of the funny speckled pattern.
I'd say to the owner that's a sign of overheating which is the worst thing you can do to an engine and make an offer based on that.
You want to know if the engine is cold before starting. If there's a puff of blue smoke at start up or deceleration it could mean oil from valve stem seals, smoke on acceleration could mean worn rings.
I'd look for signs if the mileage is genuine.
