the reason seems to be missing from this thread, so let me add it:
if there is trading outside of group play, then item drop rates have to be completely crap to compensate.
currently, we enjoy 1-12 very high end drops per run for instance. but imagine instead of having to fish for the right piece of gear, you could go to the trading mechanic (be it auction house, private out of game listings like ebay, etc) and get the exact item you need because there would be 100,000 other people that are also farming and didnt get what they wanted, but did get what you wanted.
thats exactly why gear sucked donkey when D3 launched… because anything above yellow quality had to be so rare as to never pile up.
even bind on equip will not help this… there are simply too many players farming and not enough consuming what is farmed. as the game is literally about loot farming, thats a huge concern.
on release, D3 handled that by setting timers… IE you play for 14 real hours, and you get 1 legendary drop. considering the average length of a modern game is about 30 hours of time invested, that simply wont work. (unless you want a really small target audience)
personally, i think i would be happy with something similar to how PoE handled it… the item itself has power, but more importantly is what you add to it, that you have to create yourself that is non-tradable.
imagine a system where people would trade for looks or base weapon frame, but then the player receiving it creates items to imbue the weapon after the fact. IE, its a nice sword that dropped, with a nice basic damage range, but its “white” until the barbarian imbues it with the power of Bul-Kathos, or the Crusader the power of Akkan.
players could even have a system of building up favor with their Gods, or with their crafting skills (or the skills of the crafters they hire, no reason the player has to be a master blacksmith) and then spend into the weapons and armors they have collected to power them up.
that could also be an interesting way to handle the player that plays all the time vs the player with limited time… IE the player that plays all the time would have a full bank of power, and with exponentially increasing bonuses, they may have something better to show for their time investment.
take something like hit chance, lets say the range an item could have would be, 1-25% (no other items would grant hit chance only the item to itself) the player that plays all the time pays the huge cost to get all the way to 25%. while the more casual player, lacking resources, stops at 15-20% to conserve.
both players can get the same result if they focus on it; eliminating any disparity of play time in the sense of final progression. the die hard player can simply cycle faster.
tl;dr add cosmetic item trading, not end game.