jch wrote for one, do not try and switch the gamertag to your main account as soon as you get it, wait a while.
it can be taken quickly by the dude who's selling it to you. ask him if he got the gamertag from an exploit, because if he did you can't swap the tag to your main account.

this, you really dont wanna risk getting turbo'd at all, pay with goods and services and you should be pretty safe