There isn't nearly enough gear or weapons in this game for a trading system. Everyone would have what they want and no one would play.
I'm sure most people continue to play in order to receive what they don't have.
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But doesn't that just show that there needs to be a significant improvement to the quantity and variety of loot then? Reskins, greater stat variety, new perks, etc could result in a significant improvement to the in-game loot.