How does that even work?
Can you only do that during a launch of a new system? Xbox live ID didnt automatically switch over??
The thing is that the xbox market essentially runs on old goods. The majority of gamertags being sold are gamertags that were originally made 4 years ago.
Microsoft occasionally resets inactive gamertags, allowing people to claim them again. The last mass reset was in 2011.
A lot of gamertags were also stolen around 2010-2012. Mostly by social engineering customer support. Microsoft has taken many steps to beef up their security since then and it is now almost impossible to "steal" a gamertag from someone. Regardless many of the stolen tags were banned or recovered by the original owners.
What you can do is stay up to date on the market and create gamertags when new words, artists, game specific terms, ... arise.
Again most of the tags currently circulating the market are old gamertags. An example of new trends for example would be someone selling the gamertag "Bobby Shmurda" for over $100.
Gamertags can fetch anywhere from $10 to $3000 depending on the name. $3000 would obviously be something like a single letter like A, B, ...