If I buy nba 2k16 for xbox 360 on the marketplace, then later get an xbox one, do i have to buy the game all over again? Or will it be free to download onto my xbox one with my live account.
You have to buy the game again.
Xbox 360 & Xbox One are completely different versions. Games that are available on both Xbox 360 & Xbox One has no chances for a BC, so yes, you will have to re-buy the game when you get an Xbox One.