Trading would be fine but only for things like weapon parts, other materials and maybe even vendor legendaries. Otherwise, it would result in things being too easy to obtain (even easier than they already are). And to counteract this, Bungie would then reduce drop rates so that on average we're back to where we started except now we are screwing people who don't meet the trading criteria.
I get it, you guys want it to be easier to get the things you want. But that is exactly why it shouldn't ever happen. And that doesn't even consider all the ways these various trading ideas will be exploited. For everyone of these ideas people think of to allow trading, there will be ways that it will make the game more toxic. This idea, for instance, would result in people joining massive clans so they can trade. The "only trade with people who were in there with you when you got it idea" would result in LFG posts that require you to already have Gjallarhorn and be willing to trade it if it drops. If you're clever enough to think of an idea for trading, you're likely clever enough to find ways to exploit it. And even if you're not, someone else will be.
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Maybe Trading Strike, Nightfall, ToO and Raid Drops between fireteam members would work like @So Rexy said. But they would have to be your friend, in your clan and in your fireteam.
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It should just not be in the game. If trading was an option, Bungie would then have to counteract it by decreasing loot drops as we'd be back top where we started.