With the rare Shonen Junai Gumi/GTO-tier exception, the viewership of sequels follows a very solid rule of thumb; fewer people tend to watch the second season, and the ones who do tend to be ones who are generally in love with the franchise. But not everyone who loved season 1 of a given show will end up watching season 2. In real life, stuff like time constraints, stress, and other shows all serve as potential distractions from continuing to support a franchise. Though people who buy hard copies of a show are generally in that group of hardcore fans, they’re still human, and any number of factors could cause them to keep their cash in their wallets. If myanimelist rankings tend to overestimate the quality of a sequel, then sales might tend to underestimate its appeal.
So it’s worth asking the question; what percent of its sales does a typical show “lose” when it moves on to season 2? Since I already had a list of series that got sequels from 2005-2012 laying around from my work on the sequel probability equation, and most of those sequels have been at least partially made by now, it’s a fairly simple question (barring one wrinkle) to address.