I guess it COULD work with a system similar to the one WoW uses. Loot that you get for finishing an activity (e.g. raid, strike, crucible) can NOT be traded, since you "earned" it. I know, RNG makes it look like it's not really earned, but that's nothing compared to random engram drops. These things CAN be traded, since you didn't HAVE to work for it (could've been a drop while rushing trough an area to farm spirit blooms etc.). All within an AH, not a direct trading system. IMO that could work out well.