I'm astonished though that people sell ero fan manga from popular series at Komiket, like Sailor Moon (you know... "I'm Usagi Tsukino, I'm 14 years old") [1]
[1] though there was the episode that didn't get released in the US where she used a magical item to transform herself into an adult so she could go in a bar and get a drink
parody work is super popular-- people really love to see porn of their favourite characters, age/gender/year of publication be damned
It doesn't surprise me that people like it, it surprises me that they can get away with it in the open.
i've never needed to present ID in any mandarake or book-off or the like. of course if you go to events, they'll check ID to make sure you are 20+, or i'm sure if you look young. point is, the content is just there, even in some konbini, and it's not the norm, but certainly isn't demonised-- look at maid cafe and idol culture. /shrug