In an ideal world we would be able to trade everything without any negative affects to the game.
Unfortunately, if you allow things like legendaries to be traded between players, it creates black markets for the game through 3rd party websites like d2jsp and essentially makes it P2W.
This also creates bots that then sell gold or items etc.
Personally, after experiencing online auction houses and botting a lot over the years, I have realised the importance of soulbound, account bound items etc. I think there should be as little trading as possible. Certainly no big items like legendries or unique’s.
I would make it so players have a few seconds to pick up their gear off the boss after it’s downed, if not picked up other players can then pick it up. Legendaries can be traded to players that were present during the entire dungeon run for a short limited time. Not just turning up for the boss kill (this would also create people buying and selling runs which is also not playing the game, and if they have a way of buying gold could also become p2w).
So my question is, can Diablo 4 have any trading in it? If so, what trading could or should be allowed?