I don't really think the loot system in this game is varied enough to warrant trading. Most if not all Legendaries can be purchased through vendors, and in time all Exotics will probably go through Xur. I would agree with you if we actually had some kind of deep loot system, and crafting system, but the game in the state it is now just doesn't have the gear to support deep trading.
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You're super wrong. All of the purchasable legendary items have variants with different stats. I.E. Dead orbit is the only faction with legendary boots that allow you to carry more hand cannon ammo. Unless I get an legendary boot engram that gives increased hand cannon ammunition, I have no choice but to buy those. My friend for example haS the Crucible variant of the legendary hunter boots that do increase hand cannon ammo capacity. You can't buy those. Then there are the legendary weapons. FWC is the only faction with a purchasable legendary hand cannon, but my friend got a dead orbit hand cannon in the mail once reaching rank 4. There's also faction shaders, faction exotic class gear and even faction vehicles.You CAN'T BUY THOSE.
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Even so those are just minor variations and don't really count towards have a deep loot system. I think the variety in this game is laughable right now, but I have hopes that will change
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They're not minor if you rely on certain weapons.