Ok, so if I've managed to get your attention with that headline, hear me out:
I've figured out how to add trading between players without breaking the game, or, at the very least, minimizing the unfairness.
Got your 70th piece of waterworks equipment while your buddy who's the same school and has none of the gear managed to pick up that mastery amulet you wanted? Did your friend who doesn't craft find an amber and it'd be the last one you would need to craft that spell?
Ladies and gents, I present to you, the Loot Table!
Hear's how it works: The Loot Table spawns in the center of the room after the last boss in the dungeon (Such as Malistaire Drake or Omen Stribog) is defeated (This excludes optional bosses such as Cronus). Each player has the choice of interacting with the table; you do not have to participate in the trades if you do not wish to. Those who do are taken to a GUI depicting their portrait and those of the others who enter, and are shown a list of everything they acquired since from the dungeon, excluding gold, xp, and (where applicable) spells, for obvious reasons. They can choose to put these things on the table, which anybody can take. Anything you put on the table is fair game for the other people. Anyone can put anything acquired in the dungeon on the table, and anyone can take anything acquired from the dungeon from the table. If all players leave the table while there is still loot on it, the remaining loot is randomly assigned to each player. This prevents and/or mitigates getting back an item you intentionally tried to get rid of.
This is actually pretty good and similar to what I suggested in another thread about optional "shared group loot." Regarding competition with the second chance chests, I doubt whether seasoned players actually use them as often as one might think, considering that they are crowns vacuums. Besides, the second chance chests would still be as before if you solo or if, in your group, none of you got any particularly good items. Other than that, I think this would work fine and might even be a little better than the Need before Greed system in some other MMOs.