So I saw this video on the forums. It's a great video overall but a lot of people ask why we don't have trading and at 8:41 in the video he tell's you pretty much why. Overall a great video might answer some things you were wondering about as well.
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Destiny doesn't need trading because there aren't enough weapons or anything much class specific to warrant it. Most games with trading have thousands upon thousands of items while destiny has maybe a couple of hundred.