Negative trade feedback

never.
why?

When you have effectively traded with the person and consider they are not a good person to trade with

maybe the way you got the game. I received a game through the steam trade system
and it sucks. I received it but don't even know how i did it. Gifting is better

Bro, every person is free to back off from a trade. The only reason you should give negative feedback is because after reaching an agreement the person disappears. But if you talk and he clearly says he has changed his mind, it's his right to do so.
You might be displeased but rememeber that one day you might back off from a trade too. Just be polite and say it instead of ignoring the trader ;)

Thanks for everybody's input, I got the info I needed, closing the thread.

Cheers!