If you're fine with the them going to buy the scrolls without explanation, why care with an explanation? The DM is just there to adjudicate the action of buying the scrolls, nothing more. A player might say "Hey, everyone, earth elementals are vulnerable to thunder damage." But there's nothing there for the DM to do.
Because intent matters? The narrative weight of actions can change depending on the intent behind them, and require different adjudications?
Really, the entire point of the example has been to show that players can take actions with player knowledge beyond just simply attacking something in combat.
Maybe they buy items specifically to defeat an enemy they have never researched, maybe they break into the shop to steal a wish scroll they only know about because they read the module, maybe they use knowledge from the books to confront a powerful being in disguise as an old man and use a clue they were supposed to get later down the line to trick it into fighting against their enemies.
There are many ways in which players can use the carte blanche to know anything with no restriction to disrupt the game. And the GMs job is more than just adjudicating actions, it is making sure things run smoothly.
And, while this is amusingly ironic, you seem to be fine with it on this end of the spectrum, but on determining things about a player's past and the people they know after the game has started, you are not fine with it.
I don't say "player knowledge = character knowledge" though. I'm saying the player determines what the character thinks. A player might know that, assuming the DM hasn't changed anything, earth elementals are vulnerable to thunder damage. He or she may say the character thinks that. Or he or she may not. It's up to the player. A player might choose to establish some other reason for buying the scrolls that is unrelated to thinking anything in particular about earth elementals, too.
You are giving the players the freedom to choose how much of their knowledge the character has, mostly I think because like Elfcrusher you find the idea of pretending not to know something distasteful, so do you expect players to not utilize any scrap of knowledge they have?
Except I just told you I do warn them? Through telegraphing, remember?
How exactly do you telegraph that the item they read was hidden in the fort isn't actually there? How do you telegraph that hags don't eat children to give birth to daughters?
Sure, you can telegraph something is weird about an earth elemental by saying it is blue instead of brown, but some aspects of knowledge are going to be nearly impossible to telegraph without just outright stating that you changed something.
And, I keep trying to make this clear, I'm not only talking about combat and combat strengths and weaknesses. I'm talking lore. I'm talking knowledge.
In fact, here is a good table example. We were playing a game, and we were going through a dream world dungeon full of various undead. We encountered a pair of vampires, a married couple, who had no idea they were vampires and in fact had been turned into vampires by some weird stones. One of the players, despite these NPCs having no idea what was going on and having never harmed anyone, attempted to dominate and destroy them. They were acting under the lore that all undead are made from portions of the Negative Energy plane, that they are anti-life and therefore have no rights and must be destroyed absolutely no matter what. They got upset when the DM had no idea what they were talking about, because the DM was not only not acting under that assumption, but had no idea that assumption even existed.
It ended up causing a massive fight and hurt feelings around the table, because the player went forth thinking everything they knew was true and the DM had subverted that without intention, and so while they were seeing abominations to be destroyed, other members of the party say victims being persecuted and we ended up in conflict. And not interesting party conflict, the type that nearly wrecked the campaign.
Going forth and allowing players to believe that everything they know about the game applies and is valid for them to draw upon can be a dangerous proposition. Especially when it conflicts with what the DM or other players know and are drawing from.
Great - though I don't say that "I change things constantly" as that would not be accurate. But that I can change things at all is sufficient warning to be vigilant.
Only if you change a lot, otherwise their knowledge being inaccurate is an anomaly not something they will learn to look out for.