Sorry dude its not going to happen, the loot system is too shallow and items have too little variance to facilitate a decent economy.
It would literally boil down to trading X exotic item for Y exotic item, which is stupid because of xur and nightfalls practically giving them away.
Don't forget the guy selling his vex mythoclast for 50$ on some website
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I'm sure there are people who'd pay a lot more than $50 for the Mythoclast. You should see how much the highest tier of the latest raid gear sells for in WoW.
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I was just making up a random value. I remember when artifacts in UO sold for hundreds on eBay. Back then you could sell virtual stuff there.