What about treating the other person like you'd want to be treated in a negotiation? If you're working on a deal with someone else involving the same players, say so.
This scenario would irritate me:
You: "I'll give you X and Y for Z"
Me: "Ok, sounds good"
You: "Well, maybe not, I'll have to think about it."
Me: "Dude, you offered me the deal."
Now, if, instead, they came back with, "I like that deal, but I just heard from someone else regarding players X and Y that may net me more," then I get it. Still not thrilled about it, but I get it.
If you offer a deal in a trade chat, then want to renege after the other party has agreed just because you changed your mind, at least apologize and admit that you're douche for doing so.