I only said that trading would break the game, because the vast majority of the game is spent trying to create the right build. If you could trade between characters, then it would be so much easier to aquire the top items (they are not too rare, so basic economics would mean there is a low price) that most people would probably lose interest. Tl;dr: trading would de-incentivize the grind-fest, and allow players to circumnavigate end game content.