How is trading game-breaking? If all the game wants to drop me are a few specific items due to the weird RNG algorithms working against me, trading would correct that. The game has seriously given me 4 Invectives. Literally every Exotic Special Weapon engram I've gotten has turned into Invective. But I know people who can't get Invective to drop to save their life. Trading would bridge that gap. A simple fix to keep it from being a source of boosting would be to make it so you could only trade weapons of the same rarity and year to other players. Exotics can only be traded for exotics, legendaries for legendaries, etc.

You still didn't explain how Trading would break Destiny. There are plenty of games with RNG and a trading/economy system in place. Not a single one of those games is broken, only improved. Diablo 3 didn't work because it involved real money, not in game currencies. Even without RNG involved, being able to trade items would immensely improve the social aspects of Destiny. Out of heavy synth? Maybe a buddy of yours could toss you one in a firefight. Need Weapon Parts? If only those players who have way too many could trade them to those who don't.