I offered iTunes as an example, not because they offer the best possible quality, but because they offer a frictionless, easy, impulse buy ready (cheap prices) experience -- something that doesn't exist in porn retailing.

The closest thing are the file sharing sites.

The genius of iTunes, from the very start of the store, was the idea that they were competing with *free* and thus had to give users a compelling reason to buy from them rather than to download for free. They succeeded. No one in porn seems to have even gotten the idea.