The problem is the system on its own, to many things eather worthless or farmed to often also, undercutting way to fast with the amount of people, especially with things like retainers all bringing apperently the same stuff making it also worthless and undercutting, the only stable market i seen so far is Guild Wars 2 from the games i played, sure it also has some worthless stuff but not in that amount as we have it here, but that is due one simple reason, they kept things simple.
I mean we goten countless new resources for each addon now, like 50 different armors for inbetween leveling, different types of things, sometimes goldsmith makes rings, sometimes leather or so. The amount is simply to much, on top you can't make use of everything, you can't depart all since desyncing is limited to 3 crafters, only specific crafters or gathers seem to be worth it, retainers jobs not makign a difference except gathering something specific, let alone the fisher has like 1000 of different kind of fishs by now you loose track, making people not care for what is worth any or needed for something else becasue way to many exist.
Making old materials usefull again for newer stuff in a balanced way, making able to desync everything without limitation. I mean we got things like i think one of the Obsidian it was is needed for one whole item in the game, i mean seriously SE, putting materials in that is needed for one whole item is ridiclious stupid.
Overall we would need a rework of handling items overall, making everything useful in some way simply, it will be hard of course and i doubt theywill ever change it sadly or at best which never will happen tho is remove the amount of things and simplify it, yet it works for abilitys which of course is way different but come on, a MMO is always updating and changing, if things like WoW change the world with Cataclysm as example making the old world and old dungeons lost, that is kinda the same and no one should cry if we also loosea heavy amount of useless items, i mean seriously, who needs new gear every two levels on top of dungeon items aswell.
It is simply way to much with way to less options to deal or handle things or make use of it, causing problems like the undercutting.
Side note, problem for me seems also, jsut personal opinion, that we have the typical Gil from Finalt Fantasy games, now difference in payment, so numbers go way to hight at times since no thigns like lets say copper and silver as lower money. Because for me 1 Mil Gil not feels like much since on the other hand you got so much expensive stuff like 60 Mil for a Music scroll or so or a minor gear part.