Even if it is not possible to return the item when someone logs off, something as simple as a check box where players agree that they are just borrowing would help out a lot. Reason being, if someone is scammed by letting someone borrow their items there is no way for the GP to decipher if they were actually borrowing items. With a borrow agreement check box the GP can go back and easily check if that was the case. Something like this would rid of all these lending scams.
If this is not possible to do or is seen as unnecessary than I do advise that any player involved in a lending trade run a video logger of any conversation you have with someone leading up to your trade and the trade itself.