When you want to buy or sell stackable items, you have to do it one by one

In example, if you have 25 of an item and want to sell 20 of them, you have to click 20 times and do it at slow pace due to the animation triggered by each click
and the server probably receives 20 transactions to do so (assuming thin client)

If a pop up was asking how many you want to trade, it would take from us only one or two clicks and the server would receive only 1 transaction to fulfill the request

(with thick client the qol part still remains)

see you