I've seen this issue. It says sell for 100, you go to sell multiple and the trade price changes completely. You go back to trade 1 and it still doesn't sell for the described price. you hit reset and it shows the described price again. You can sell 1 for that price, but if you try to sell multiple you don't get that price at all. Not sure if I even got that price on selling 1. It's one of those niche issues with demand being on the edge. Annoying when a npc says you can make x gold on some, but its actually 1. You bought a stack for nothing.