All Credit goes to Otokumegane who started all this. Check out his Nintendo Tier list
http://gamrconnect.vgchartz.com/thread.php?id=155867
Also the Microsoft List can be found here
http://gamrconnect.vgchartz.com/thread.php?id=155873&page=1#
S-Tier - 10m+ Sellers
A-Tier -5m+ Sellers
B-Tier - 2-5m Sellers
C-Tier - 1-2m Sellers