We currently do not support any trading in Fantasy League of Legends, and this is intentional on our end.
Since your roster positions are not able to be modified and you can only have 5 players, we decided to disallow trading because you would only be able to do 1-for-1 or 2-for-2 trades, etc, and they'd have to be of the same role.
For the first split, we purposely disabled the trading to avoid users acquiring multiple players of the same role, which would cause a roster issue.
We may consider adding trading to Fantasy LoL in the future, but right now it is not possible.
If you'd like to conduct trades manually within your league, the best way to do this would be to have the commissioner use the commish tools to force drop/add players to and from teams.