What do I need to know about buying a used bike?
Things to think about
The condition of the bike is more important than how old it is or how many miles or kilometres it has.
Adam at clarity.net has written this article, which has been used by a lot of people. It should help you look at a bike and evaluate its condition.
Please remember this tried-and-true advice: If you don't really know what you're looking at or for, don't be afraid to pay a good mechanic the $100 or so to give it a thorough going over, usually known as a pre-purchase inspection. It's your life.
Checking vehicle history
One thing you should do before you buy is check the Vehicle Identification Number (VIN) against a database. There are a few different ones.
After you buy
One of the most important things you should do, before riding it, is do ALL the normal maintenance. Assume that the previous owner did nothing. Go down the Service Schedule point by point and make sure everything is up to spec. This process includes checking the valves, syncing the carbs, flushing all fluids, cleaning, inspecting, and adjusting the chain, and more. Also, check for any time-related maintenance items, such as replacing the rubber brake lines every four years. Doing all this stuff will help you get familiar with the bike and find stuff that's broken or messed up and therefore dangerous. This will also allow you to start off with the bike in the best condition.
If your used bike has been sitting for a while, see here.