Unless the publisher releases sales for the online versions, it's difficult to track.
mrstickball has taken it upon himself to trace the XBLA leaderboards and estimate sales for games. I imagine you could do the same for PSN releases? Not sure about VC/WiiWare.
Threads of Interest:
The Movie Thread: http://www.vgchartz.com/forum/thread.php?id=6880
The Crow Eating Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=3886
The Betting Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=7104
Custom GIFs Thread: http://www.vgchartz.com/forum/thread.php?id=18963
The Greatest Game Ever Conceived On Any Platform
Tag: "I have tasted Obi-Wan's bitter tears"







