It would work if you could set up a price schedule and keep people honest. It would be someones full time job to act as arbiter of the warehouses. IN ATITD there was a commodities index of what was worth what. As the amount in the warehouse increased the good was worth - money. As the amount in the warehouse decreased the good was worth more- money. So you could trader 120 iron ore at 7 q for 60 copper ore at 35q - if that is what the spreadsheet said was available, but then you need someone to do the transaction.
Could it work? Yes. It would be great, but why limit it to just the guild - it would be a worldwide exchange at a capital city. Why stop there? You could open a website and open it up for Real Money Trading. Then it would actually make cents to devote time to being a bonded trade agent. The warehouses would be a pain to manage and you could pay people with in game coinage. In game coinage could be exchanged for real money. Not sure how that would work with the company though.