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.
I feel like you could have powerful dungeon tier items be BoE if they're 1. Craftable using a rare recipe with X amount of crafts per scroll 2. Uses materials received from the dungeon itself or by scrapping dungeon BoPs and BoEs.
If the game could have a crafting system that allows you to "borrow" other player's blacksmithing level to craft Bind on Pickup items, you could even have people craft replacement items that are either equal or maybe even slightly better than the dungeon equivalent to cover for their bad RNG as long as they've still gotten some of the material drops/scrappable items from the dungeon.