I have previously expressed my frustration on this Board about people who let trades hang for weeks and who never bother to set up their Manager Options or Lineups for away games or to play their own games.
It was pointed out to me, probably rightfully, that not everyone has the same amout of time or interest in the game and that we need those players too to keep the game going.
To answer your question about how to handle those trades that hang forever, it depends on the time of year. Right now, with nothing much happening and a lot of people just coming off holidays and perhaps busy or focused on other sports (I hear there are other sports, though not for me!), I give a long time.
For example, between Dec. 19 and January 1 I posted a number of offers for my different teams. Granted, these were all blind offers, with no email accompanying the offers. While I think all of the offers were at least minimally reasonable, some owners may have considered some of them insulting.
I heard immediately from some owners and have either traded with them or abandoned the effort because we can't get together or because we're waiting on such things as seeing the Rookie Draft order, seeing who's available in the draft, or maybe even seeing what given players do in Spring Training.
This weekend (from one month to three weeks after I posted the offers), I went through and withdrew a couple of offers that, in retrospect, might have been one-sided. With the offers I still think are reasonable, I emailed the owners, reminding them I had the offer out there and telling them if I didn't hear in a week, I would withdraw the offer.
I do this because if somebody withdraws an offer on me, I like to know why. Actually, I don't expect to hear from any of those guys now if I didn't hear from them in three to four weeks, but I just think it is common courtesy.
Of course, during the season, the timetable for this would be shorter. I also think it is better to accompany any offer with an email, because you usually at least get an acknowledgement of the email.

As to people not looking at the other teams' roster, I have had that happen, but some people are such active traders that I believe they think if you have a star, you can turn him for what you need.
Sorry this post is so long, but this is a hot button issue with me and I expect with a lot of us.
Dave