This is my opinion and feedback based on my own personal gameplay experience.
It has become common knowledge that the trading post does charge a 5% front-end fee to sellers as well as a 10% back-end fee to the same seller.
I do feel that guilds that players are in should receive a portion of those fees to support guild operations. For example- I would say 1% of the front-end fee goes to the sellers guild (5% is still taken, if the player is guildless then no guild can claim and the 5% is taken out all together).
How the system handles the rest would be up to constraints of the current system. If the money is meant to be taken out of- then the guilds can receive credits that can be used in lieu of actual coin for upgrades to the guild or upgrading locations in WvW.
Engineer – PRX – Hedge of Denravi