This works like the old D2 trade hack scams where the recipient would see an item's image in the trade window, but wouldn't receive the item upon accepting the trade. The bug was created when a dupe method using similar, but a few more steps was recently patched.
-This only works with stackable items
-The procedure will describe how Player 1 will trade hack scam player 2
-Player 1 has a stackable item divided into 2 stacks of any quantity > 1 side by side in the same bag in their inventory
Player 1 opens a trade with Player 2
Player 1 right clicks both stacks quickly.
-Both stacks will enter the trade window. The 1st stack will be locked and cannot be altered in the inventory. The 2nd stack will not lock, but will be in the trade window.
Player 1 right clicks on the 1st stack in the trade window to remove it from trade
-Now, only the unlocked stack is in the trade window
Player 1 splits (n-1) amount from the stack of n items remaining in the trade window and places the items split off the stack into another inventory slot.
-Example: a stack of 20 items is in the trade window. Player 1 would split, from the unlocked inventory icon, 19 items and place them in another inventory slot.
The trade window's stack value will not update for Player 2, so it will still display n items in the stack, when there is only 1 item left. Accept the trade, and only 1 item will be traded to Player 2.