I think personally an auction house is not what this game needs.
My idea is the following, it's not complicated or hard to implement unlike an auction house:
Instead of an auction house there is just a mushroom left behind at that particular spot.
So the mushrooms stay but you can move freely... and if you leave the game the mushroom also goes away. (like normal)
And just create a vendor buf or something that when you delete it that mushroom closes and gives back items and money.
The only issues that would bring are the following ones,
Inventory when full will create a lot of issues and ofcourse inventory usage.
So what they should do to implement this is keep all the old systems and make it so after putting your items a mushroom will pop down and you can walk away.
The placed items will be locked and unmovable in your inventory until said mushroom closes. (like the old system but only affecting the items you put in)
You can also directly change item prices or close the mushroom by clicking on it.
After you put a mushroom down you will get the vendor buff, after removing this buff your shroom will go away and items or money goes back to your inventory.
In game text messages after something gets sold.
Remote acces to check store by pressing the vendor icon in game, option to change prices, close.
Old system but you can move freely and only your sold items or items you buy get locked.
I always found it strange why this was never implemented. Why should someone open a separate account and waste energy, performance over one simple store function.
Some people have toasters like for example honeyteddy and being able to just setup a store on one account and being able to play on it aswell would improve their overal gaming experience a lot.