I agree with Lobo, that if the other person is being a pain to deal with, or rude, it is better to just take a pass and look elsewhere. It sometimes means you have to wait a while before you find the piece you're after up for trade again, but that's better than getting aggravated. I've had a few trade discussions like that, and I'm not going there any more.
As to the matter of not responding to weak or bad offers - I always reply with at least a "no thanks", and sometimes a counter proposal. It sucks to be left hanging, and I don't like making multiple offers with the same piece all at once. Sometimes I'll include a time limit in my offer, after which I will begin offering the piece to others. That takes care of the really slow responders and those who are online infrequently.
My 2 cents.