There is a safe way of trading between characters that prevents one character from cheating another character. When you want to trade, click on the other character and on the Misc Tab click on Trade. That opens the following window for you and the other character: This should be the only way for you to trade goods. You should not exchange goods by dropping them on the floor since you never know if the other character is true to his word.
Actually, in order to open a tradewindow, there is a little handshake necessary to prevent malicious players from opening a tradewindow on other players in a stressful situation like a fight:

If another character opens this window for you in order to annoy you, you can turn off externally brought up Windows by hitting the F8 key.
You can also click the button in the lower left corner to close the window and prevent further trade windows. You can re-enable windows later by hitting the F8 key again. You have to do that in order to accept any foreign windows like the trade dialog or the Surrender dialog from a guard in case you are a criminal.
The window itself operates quite easily. Double-click the objects you want to offer in a trade in the top list. They move to the second list to show you, what you have offered. Similarly the third list shows you what your partner offers. When you both are satisfied with the offers, both must click the Accept Deal button. If one player clicks the button and the other character changes the deal, then the deal becomes invalid and both characters must commence the trade again. This makes sure, that both characters agree on the exact same offer. The goods are then exchanged and that's it.
There are three peculiarities in the trade window. If you want to offer a house, you have to check the check box at the top so that your houses are shown in your goods list. When you offer it for trade there is an additional confirmation box that should keep you from offering a house accidentally.
When you want to offer money, you enter the amount you want to offer in the Money - edit box and click the Confirm Offer button. This sends the money offer and shows it in the other characters trade window, but it does not send the money itself. This only happens when you click the Accept Deal button. The Confirm Offer button is nothing else than a signal to the client to show the money you offer on the other client.
Finally, when you trade house shares, you can not offer anything else in this trade. The other partner can only offer money in return. This ensures that the server can determine the value of the shares and credit each share with its monetary value.
If you change your mind and don't want to trade, just close the trade window with the button at the bottom and the trade is ignored.
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