If I (accidentally) left-click on an item while at the armor & items vendor, the item gets purchased even though it looks like I’m just picking it up. When I go to put it right back down (left-click again), the item disappears (because it sells the item). Then I have to pay full price again to retrieve it from the Buy Back screen.
This has cost me a fair bit of gold (and lost items I was considering buying) before I figured out why the items (and my gold) were vanishing.
Since the game tells us to hold Shift and use right-click to purchase, this can’t be intentional behavior. If it is, it’s a poor user experience.