(I am playing on Xbox One)
For me, pressing X and Y to sell a stack does not work. I have to click on the stack and when the popup screen asks if I would like to sell the item, I have to move my cursor over and click on in X. (It does not work if I click B, I HAVE to click on the cancel option on the screen.) Then I have to click X so it goes back to my cursor, then I hit B and it takes me back to the stack. Then once I click Y to sell, I am finally given the option to sell the entire stack.
To sell items on Xbox One in The Sims 4, hover over the stack of items or item that you want to sell, and proceed to press . This will then bring up a sell item menu. Hope this helps!