When I shift-left click on item in vendor window it disappears.
After some panic I discovered that when I shift-left click I buy the item and immediately sell it back to the vendor:(
I know that shift-left click is not official key combination but it is easy to confuse shift-left click and shift-right click, especially if you play other games at the same time as you play Last Epoch.
Also the so-called help icon on the bottom of the screen that explains you should you use shift-mouse button is ambiguous it can mean left click as easily as right click!