Something to identify it as youre bike would be good, like name and phone number. But even better is if you note the bike's frame number. If the bike gets stolen and found somewhere it'll make it easy to verify that it's yours.
It is fairly easy to sell an item online through the internet. I think you should list your exercise bike on Craigslist. All you need to do is take a picture of the bike and write a description, and list the price you want to get.
You should get a bike license if you ride a bike and don't want a citation.
as light as possable
Both.
bike
depends on the bike.
i was unhurt when i fell off my bike?
theft report
The bike that you should think of buying an active 9 year old is a 24-inch wheel bike.
when you sit on the bike, you should be able to touch the ground with your toes.
That depends on what you are using it for. Obviously, if you are dirt biking, you should use a dirt bike, and otherwise use a street bike. Start out with what you are going to use the most - dirt (freestyle) or street racing.
A ballpark value is that a used bike should be half the price of a new bike, if it's just a year or two. An older used bike should be even less.