First, I have been a software developer, development lead and product designer for over 26 years. This means I have more experience than everyone at ZoS. I even have more experience than most of ZoS ESO devs have been alive, going by what age they appear to be in the videos they release. (They are young'uns!

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Second, everything I am about to post can be Googled.
Third, If you think I "don't know what ESO's software stack is, so I can't possibly judge if something is possible or not without knowing how the ESO server code is written", you're simply wrong.
Lets get to it.
Any software can use more than 1 technology. I could have a client/server application (ESO) written on a certain stack (lets say C#, java, MySQL for example) and even build it into a proprietary "engine" (an MMORPG engine). That does NOT prohibit me from adding a specific game feature which uses a completely different technology than the main application, especially if that feature is what's referred to as "modular". i.e. if it doesn't really have to integrate very much with the rest of the software. For example, I could add (to the stack I just mentioned) a feature which uses a "PhP/JQuery/Node/MongoDB" stack, and integrate that into my UI pretty seamlessly.
Now that we've established that, no matter HOW ZoS has coded ESO, I could add anything I want, in any technology, to the ESO game client easily, no matter what technology it needs or what tech stack ESO already uses. Got that? OK.
Global Auction House: ZoS says its "not possible due to the load it would have because of the # of players (due to using single megaservers)"
I have two words for that claim: Bah. Loney.
Using a No-SQL DB I could code a global auction house for ESO that could support 50 million players, each posting 1,000 items for auction, and you could search it and post bids in less than 50 ms (milliseconds) for each database access. In other words, any number of players of ESO could use this AH with no lag whatsoever, even on a megaserver. Again, all of this can be Googled if you so desire.
Conclusion
ZoS's reason that a global AH isn't possible due to performance reasons (due to using megaservers) is not a valid reason. The only thing preventing ZoS from having a global AH is to want to provide one, and to be willing to design it with proper technology which they can plug into their game easily.
My Recommendation
I actually do not support "Auction Houses" (i.e. players post items with bid/buyout prices, other players bid on item until the auction expires, etc.) Instead, I support what ZoS already has for guild stores, which is a "store' concept, just at the global (game) level. Here are my rules for such "Player Stores" which any good MMORPG should have:
- Players list items for sale only (no bidding system at all).
- Rigid categorization helps scaling: i.e. "Swords", "Maces", "Heavy Chest Armor", "Light Boot Armor", "Necklaces", "Rings", etc.
-- No ability to browse/search
across these categories (i.e. you have to search "Swords" for "Lightning", then search "Shields" for "Lightning", etc.) It puts a little more onus on the player to do multiple searches, but you could mitigate this by some good UI features like preserving the search term as players swap categories so they don't have to re-type it over and over, even preserving search terms across sessions. Also, lots of good pre-set filters like rarity, material, quality, etc.
- Game-enforced price caps based on rarity of items to control/eliminate market exploitation and inflation.
- Account-level posting caps to prevent overloading and exploitation (for example, 10 items max for sale at once, per account)
As always, I welcome discussion.