Usually there is an unwritten rule that all trades should be fair and should benefit both sides. If a trade is sent to you, you have the option to accept, reject or make a counter offer to the trade. If the trade is accepted, it goes through a "review process" where the trade is visible to everyone in the league.
If you do not like the terms of a trade for other teams in your league, you can protest the trade. If a majority of the league votes in protest, the trade will be denied.
Below is an example of the screen you may see to conduct a trade if you belong to Yahoo Fantasy Sports.
You start off by choosing a player from your team and then choosing what you would like to trade for. You can search what you would like to trade for by position, by name or by team owner.
