While looking on MangaGamer's website, I happened to notice that ef - a fairy tale of the two was carrying an actual ESRB AO rating on it, as opposed to the usual 18+ sticker. Curious, I looked to see if it had been officially rated.
Yep. "Adults Only 18+: Content Descriptors: Blood, Nudity, Sexual Violence, Strong Language, Strong Sexual Content."
I thought this was noteworthy as very few visual novels are submitted to the ESRB to begin with, and an "AO" rating is very rare for a game to get. Perhaps it could be a selling point? I know that Seduce Me, some sort of trashy sex RPG, sold quite well on Amazon for quite some time due to its AO rating.