The current system hurts people way more than it could possibly do if you could trade loot from bonus rolls.You can't trade loot from bonus rolls right now because the team wants you to spend that token on yourself.
I've bonus rolled several items which are BiS for others but complete garbage for me(or something i already had). The first member in our guild who got Whispers in the dark on HC managed to bonus roll it twice the following weeks..
If you are playing with a group of people who would even think to force others to spend coins to trade loot you should stop playing with them. Blizzard should not have to design the game around this.