I personally like the idea of an auction house or a stall system, both have pros and cons, there is also a hybrid system similar to ESO where guilds or large groups are able to bid on NPC traders stationed around the world and give their shop a good location and have it stand out. This can help push to create dedicated Trading Guilds and will likely force a regular fee to join these but in my opinion would force trading to be more specialized which can be good or bad depending on the overall game direction planned.
I think the Auction House is the best straightforward option, it's familiar, can accomplish what's needed and is very accessible. I don't think more than one system would be needed.
I like the idea of all items being tradeable but idk about ALL items being able to be sold auction house level. Say there is a set of items that you have to go to "Dungeon C" to acquire, late in the game if this set becomes saturated then no one will have a reason to go to Dungeon C if they can just buy it, as the value will lower over time. Items and economy can help steer players around the world to keep areas from dying.