I know how to solve (chinese) farmer hackers and mastery book issues

A scroll NPC would not only get rid of hundreds of millions, even billions of mesos from the game,

Try hundreds of billions on the lowest of low estimates. And the meso centralization flows towards the rich, making them comparatively richer, as locked stated.

So what you need to do is let the scroll prices fluctuate to match demand. A simple way to do that is to have the price depend how long it's been since someone bought it, and then update the 'natural' price every day. So, say you buy it within 1 hour of the previous buyer - then it's sold for 100% of the day's price. If nobody buys it after 1 hour, it cuts to 50% - but you can only get 1 scroll at the discount price, then it's back to full price until another hour later. Wait 2 hours with no buyers, it cuts again, to 25%.
At the end of the day, the average price paid for the scroll is gonna be somewhere between 100% and 25%. If it's way less than 100% (say 80% or lower) then make tomorrow's 'base' price lower. If it's at 100%, indicating people bought them steadily, then raise the price tomorrow. This means def scrolls and other less valuable stuff drops hugely in price, while things people actually use (gfa, armor luk/str/dex/int) stay worth something.

Yeah and put the guild style merchanting on auto-pilot. To any affluent group of people who have a damn clue or ounce of foresight, this is free money unless scrolls bought are untradeable. There needs to be limits on how many one can buy for this to work at all that or the untradeable solution which is also terrible.

Teachendemeconomics.

A more plausible idea in my opinion would be to include more repeatable quests that reward scrolls as awards rather than straight out mesos to acquire scrolls. That or insert alternative ideas for acquiring scrolls.

Best idea in the thread especially if the quests take a pretty consistent amount of time to complete which means the rewards will not come from drops but tasks and including truthcheckers. Doing this as an addition to drops of course. Doesnt elimate merchability but also stops botters. For an extra step to preserve merching, scrolls acquired this way could be untradeable.