Buying a second hand bike is good if all the main elements of bikes are known well. Ride the bike upto a certain kilometers , look for any obstruction , hear all the sounds from the engine .Inspecting the bike is an important step.
Here are some important checklists-
Brakes- check for smooth operation age of brake pads, and make sure that there is no pulsing.
Oil leakage- look for oil spillage around the engine. If the bike has been was recently then the area with the oil spillage will shine more than the rest of the area. Always ask for the history of the service details of the bike. Look for oil leaks around the suspension
Rust/Corrosion - Look for corrosion on the bike frames. Surface rust is not a point of concern but deep rust with crumbing iron flakes are signs that bike was not maintained properly.
Clutch - Check for lever when it is released. It should not need any effort.
Chassis- The whole chassis should be thoroughly inspected for scratch marks. Look for fresh paint as that is an indication that the bike has met with an accident and thatthe paint was new to hide the damage.
Chains - Rotate the rear wheel it should rotate uniformly and there should not be any screeching sound.
Electricals - Check all lights and batteries and make sure that they work.
Exhaust pipe- If there is oil spillage in the exhaust pipe then this means bad rings and valve guide seals.
Paper work check- make sure that all the certificates mentioned below should be there -
i) The engine and Chassis number printed on the RC book should match.
ii) Insurance certificates
iii) Pollution under control certificates
iv) No objection certificates