If all gamertags that didn't exist were available let me go grab the gt "Run" real quick. You have to sell an actual gamertag not an idea of what you should make your gamertag.
1. Your stupid
2. Its taken by a Microsoft employee so no one can take it and use it.
3. Its Been deleted off the servers and put into a "limbo" state
4. Its of limits because its slang for jerking off
5. your stupid