I'm not sure how trading will increase the longevity of the game. There is not enough content to play without repeating the same stuff hundreds of times.
The only reasons to keep repeating the content are for quests and to get loot.
If you can trade for loot, then that removes one of the main reasons for playing the game.
Besides, Bungie has said no to trading. Every poll on the forum that asks opinions about trading shows that the majority of players don't want trading.
It won't happen.
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But players wont give away their hard work for nothing, you will equally have to work your ass off to afford items you want.
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Getting glimmer and other resources is easy. Getting a specific item is hard. If you can trade to get what you want you can increase the chance of getting something 3x (or 6x) for raid gear. That means less grind. Friendships are lost if people don't want to trade. Game does because people have less reason to play. No No [spoiler]no[/spoiler]