A lot of posts have used Amazon Bestseller lists to indicate the relative success of different games or platforms. Each of these posts is filled with people saying how pointless or dumb it is to look at these lists.
While Amazon Bestseller lists don't give exact numbers and cater to a different demographic, they can still give a good indication of the sales trends of different games in different regions.
For example, the Canadian Amazon has NHL 2010 for PS3 and X360 in 2nd and 3rd spot respectively... but its not in the top 100 for the American amazon.