So many times I've gone to use the currency exchange, typed in the exact number I want to sell something for, and then right as I'm pressing the button to post the order, the buy orders change, and the game automatically changes my sale price down to whatever the highest recent buy order price was, which is usually well under the number I manually typed, and I end up selling it for less than I intended and can't do anything about it.
In my opinion, this is really poor UX. I can't think of any other website, online form, exchange, or game that works this way. You almost never want these prices to be automatically changed for you. It's too risky and can result in you accidentally selling things for way less than you should have.
The least GGG could do is offer us an option in the settings to toggle this off.