To barter successfully, without being cheated, make sure you check what the other person is offering. If they previously used the chat to initiate a trade, make certain they stick to their offer, if they do not, just reject the offer and make a note to yourself to be suspicious of that player when engaged in bartering.
Do not believe anyone not willing to give what they first offered, and reject promises that they will "pay" later.
It's a dog eat dog world out there, take care.