I think this would probably be the best way to construct a Top 10 (or 20 or whatever) list -- aggregate the top seller lists from as many web retailers as possible.

From these three charts, it appears that "Pirates" and "Swallow My Squirt" should be at the top of the chart. They are the only titles to appear on all three. It is interesting that "Neu Wave Hookers" is not on any of the charts.