This is not written by me, but lifted off "The Motorcycle Rider" Delphi forum located at
http://www.delphi.com/motorbiker/messagesSome things to take note of when purchasing a 2nd hand bike :-
1) Check Brake Disc for warp and chipped
areas.
2) Steering Cone.
3) Check for leaking fork seals.
4) Check sprocket and chain wear.
5) Look out for contaminated brake oil in
the reservoir.
6) Condition and wear of tyres.
7) Wheels Bearings, shake wheels they should
not budge at all.

Check for rust in fuel tank.
9) Check suspension for too soft or too
hard.
10)Check all switches and lights.
11)Check for oily patches on engine and
visually inspect gaskets.
12)Check oil filter to see there is any
damage around the sides.
13)Coolant rubber tubings to check for leaks
14)Check frame to see if there are cracks or
re-welded.
15)Blow engine after warm up, no smoke
allowed for 4-strokers. Smoky no good.
16)Check engine block to see got putty or
silicone. Ask why is it there.
17)Single carbs no need to worry too much,
in-line-4s check for spark plug firing
and snyc.
Hope this helps.