It's simple. If you want to trade something(armor, weapons, or consumables) as soon as it changes hands any progression on armor or weapons is completely zeroed out. You'd have the piece, but you would have to grind it from zero like any other piece. The value of it would be much less. Bungievision could set it up that the only way it could change hands is if in game currency is used. They could set up an auction system as well that will only work with in game money. The only way you should be able to trade without cash should be if you are in a clan or if someone is on your friends list.