Although I think this is a fun idea. It’s not really feasible. At least not for a long period of time… I’m not going to go into the wall of text it would take to explain, however I’ll just say that controlling the market is pretty much impossible to do on your own for a long period of time and would require the strongest players in the game, to gain the amount of money it would require. In-game economy is vastly different from real life.
In-game currency is unending, and just magically appears from selling stuff to NPC’s. Items aren’t created by a “supply and demand” focused company, but rather just random drops from monsters. Currency sinks are a thing, and honestly prices are almost always dictated by how difficult something is to get, and/or how important it is.
It’s a nice dream, and maybe if you alter your focus, sticking with being a merchant guild, but aiming for more of a currency collection goal. You could then focus your guild on supplying your members with gear and items through currency. Even so, your idea is screwed if end game items become character/account bound. I’d just take a long think about HOW your are actually going to achieve this magnificent feat. You need some real outside the box thinking if your going to go ahead with this.

I like to forge. i got you guys with weapons for the low IGN: gonna be Weapon Dealer