And that it will start when the customer wants to hear it. That has potential for embarrassment. ;)
I just bought a dirt bike and when I was in the process of lookin for one, I asked my uncle what to look for. He emailed me a little checklist type of thing that might be good for you to go over. Keep in mind, this is from the buyer's point of view, not the seller's.
"Questions"
-Who rides the bike? How old, where do you ride? Was the bike raced and where do you do most of your riding?
-Do you have any other parts that go with the bike? Desert tank, hand guards, graphics kit, etc?
-How often do you change oil? Who does the service on the bike (you or the dealer)? Do you have service records?
-Have the valves been adjusted? When, how many rides since the adjustment.
-Do you have a paddle tire for the bike? If yes the bike is ridden in the sand. Sand is hard on the motors and wheel bearings. It gets everywhere and causes everything to wear out fast.
-Is the registration current and do you have the pink slip. Is the bike green sticker or red?
"What to look for visually"
-Paint worn off of frame. This means the bike gets ridden hard.
-Look under bike for dent in the bottom of frame. Small ones are ok, big ones mean the bike has been ridden hard.
-Look for dents in the rims. Fairly common.
-Look for oil running down the front forks. This means bad fork seals.
-Look at the radiators to see if they have been repaired. They break easily.
"Good things"
-Hand guards on bars.
-Triple clamp upgrades and better bars (pro taper, renthals ect.)
-Steering dampeners. New $600. Most people take them off when selling.
-Good tires. Look for square edges on the knobbies.
-Skid plates.
Hope that gives you some direction. Good luck!