Won't need to sell well. Developers are putting it onto the PC as well where it most likely will sell well...if not pirated to death.

At least the devs will know that it will sell on the 720 if it's a launch title since the 720 won't be hacked by then at launch, and just like all launch titles on new consoles, they generally sell well since the amount of games available at launch are very slim with little competition. Just look at Resistance on the PS3 and compare it's sales to the rest of the game sales in the franchise even though there were many millions more consoles on the market at the time of their release.

ArmA will sell just fine if there are limited amount of military shooters to compete against it on the 720 at launch.