Hopefully this can become a sticky thread too.
Thinking of buying a used 250 (or any other bike for that matter?) Here is a detailed list of things to check out when looking at a prospective bike:
The first thing is simply start it up....how easily does it start cold (yeah I know Ninja 250s are very cold blooded), but how easily it starts cold is a great indicator of overall engine health and whether or not the carbs need work.
Be suspicious of anyone who warms up the bike before you arrive, they may be trying to hide cold start problems.
If possible, arrange for a mechanic (or yourself if you know how and have the gauge) to do a compression test on both (all) cylinders.
Once warmed up, the bike should idle without misfiring at around 1000-1500 rpm. If the idle is set higher, be wary as again the seller may be hiding carb or cold start issues.
Look for crash damage, (chipped paint on the frame at any joints or welds is a huge red flag), and signs of oil or coolant leaks.
Is the bike clean? I mean inside the fairings and around the engine too. If the engine or inside of the fairings are caked with dirt and oil then it shows lack of care and maintenance.
Look at the condition of the brake rotors and pads. Scoring, excessive wear or signs of overheating (blue tint to the rotors) are all signs the brakes will need repairs. Look at the brake fluid in the front master cylinder sight glass and rear brake reservoir..it should be pale yellow (unless some other brand was used..for example, the old Motul 300c was blue). If it is dark brown or black it hasn't been changed as needed, and not only is the fluid shot, but odds are good that all of the seals, master cylinder pistons and brake lines are shot too. (Big bucks to repair/replace.)
Check the condition of the tires. Excessive wear, flat center profile or signs of drying/cracking means you'll need to replace them.
Look at the engine oil in the sight glass. Ideally, in a well-maintained bike is should be fresh amber looking or slightly dark. If it is deep black (and leaves residue on the glass when the bike is returned to the side stand) beware, again that shows lack of proper maintenance.
Pull the clutch lever...it should be firm but smooth. Any roughness, cable is bad. Excessive lever play could be simply due to rider preference, or mean that the cable is stretched and worn out.
Next, test ride the bike. Clutch slipping and/or grabbing is bad. How does it shift? If it is excessively noisy or fails to stay in gear, the trans is shot. (Expensive repair.) A mild gear whine is normal.
Engine should run smoothly, and not hesitate or stumble.
Handling should be firm and not sloppy or twitchy.
Any flaws or things that need fixed should be used to negotiate a better price.
Inspecting the bike should take about 15-20 minutes and the test ride should be about the same.
If it all checks out, buy the bike and have fun!