Are you equating leaderboard numbers to sales figures for those games? Because leaderboard numbers include people who have played the game that don't actually own it.
But I suppose that is the best way to get a rough estimate of the sales....I'll look up some of the games that I own and give you the numbers for them.
In this list I put leaderboards = sales.
The amount of people that appear on them with a 2nd XBL account should equal out those who play their arcade games offline. Well except for multiplayer heavy games and especially those where you can just invite any friend to play local co-op online.
In the end these are all of course just estimates as always, but I'd say pretty close ones as I don't think there is more than 10% difference to the real sales.