When it arrives at the part of the item you want to sell, you click on the item you want to sell, throw it away while it is in transition, and it still considers the item to be in the inventory, but not in the inventory! So he just clicks to sell and doesn't take anything from his inventory, considering he did!
The solution for this would be when the person finishes the sale, in the part in which they select how much they want to sell and click on the button, first take the item from the inventory and confirm that the item is really in the inventory, showing an error text if the item is not in the inventory, but if you confirm that it is in the inventory there you can hand over the money!
There is a serious bug!
When it arrives at the part of the item you want to sell, you click on the item you want to sell, throw it away while it is in transition, and it still considers the item to be in the inventory, but not in the inventory! So he just clicks to sell and doesn't take anything from his inventory, considering he did!