I'm in a league where we've been together about the same time. The thing that's tough about that is everyone has a good feel for what everyone else's tendencies are. That's probably the #1 reason I don't offer anything back, I already know it's not even worth my time. I try and upgrade my team ALL the time and will look at the depth charts throughout the league all the time, but know that there's only like 3-4 teams actually worth sending something to, because the other 7-8 guys either never look at it or won't accept anything unless they're absolutely raping you in the deal.
There's like 1-2 guys in my league who think...let's try and better both teams.