In my opinion, I think it completely depends on your league. For example, my league consists of a close group of friends. It's competition. Most GMs in my league are pretty intelligent, slightly overvalue their own players, and are always trying to win the trade. However, there are always players on a team that go through slumps and a GM thinks it's time for a change, or the player has lost value to the GM.When I say intelligence I mean it in a general way, there are various levels from superior to inferior regarding hockey knowledge/intelligence within the league.
Now to the point. Know who you are trading with. What is the team's weaknesses and strengths, how can you compliment each other with a trade. As Pengwin suggests, asking a GM which players are available directly tells you which players a GM currently values. Some players such as Vanek nobody really wants to trade for because of his incredible hot/cold streaks.
Personally, I don't make many trades per season, if at all. Last season a made a couple. This year was offered and declined Giroux and Marchand for Toews and Benn.
Hockey
12 Team Yahoo H2H standard scoring 1 year
Start: 3C, 3LW, 3RW, 4D, 2G
G,A, PPP, SHP, Shots, +/-, W, L, Shutout
To be drafted.
Football
10 team league. Standard Yahoo scoring. (Non PPR)
Start 1QB, 3WR, 2RB, 1TE, 1Flex (WR/RB/TE) 1K, 1DEF, 1 defensive player
QBs: Wilson, Manning
WR:V. Jackson, Patterson, Benjamin, Cooks
RB: J. Charles, A. Morris, D. Martin, K. Davis, K. Robinson
TE: Gronkowski, Cameron
K: Novak
DEF: Steelers
IDP:Worrilow