Buying Rare Youtube Username!

"1st off if this is against the rules please let me know and lock/close this"

I'd like to know if you can sell/buy rare youtube names. Even if you are not allowed how do people come up with some of these names?

I have seen people just recently acquire names that were taken before.

Example http://www.youtube.com/channel/UCax5HVw3GK6UgVTQPBX0NIA
(if scared of clicking links just look at "Tom Brady" he just joined august 7th on youtube.

Anyways I'm buying any rare name.