In 1.14, how do you trade quickly with Villagers? For example, when you want to trade sticks for emeralds with a fletcher, how do you trade a lot of sticks without clicking on the stick trade, then shift clicking on the emeralds. I have seen people spam their keyboard and do it quickly, but what keys are they pressing?

They are spamming the spacebar

select the trade, spam spacebar and left click. Though be careful with this when playing the game casually because with villagers like librarians, sometimes when mashing spacebar it'll swap to a different book trade without you knowing it and, from experience, it's not a fun thing to happen.