The exchange displays 400 items max of any one thing, if more than that are present they are excluded from the search. This allows players to buy or make stacks and then submit them one at a time until all other trades are off the screen.
Some players have discovered if they use multiple characters they can submit 400 or more of something on the exchange and essential blanket out stacks of items for gain.
User Interface (UI) - Exchange
How to reproduce (If known.)
Have multiple characters submit the max amount of something until stacks are no longer an option past the 400 max items the exchange will display
Don't allow players to submit more than 20 of the same thing from their character or account, 20 is more than enough to sell whatever a player has of the same thing.