Re: broker. That would allow people to turn in broken, unmendable, crap sets and swap them for shiny new ones. Not a good thing to get people together, out there and smobbing.
Re: guilds. People wouldn't even trade for 'ingredients' when these were originally implemented. I think you're overestimating people's willingness to trade for stuff. Also, there is a snag with broker, with regards to these items being available or not. It depending on other players depositing items at the broker leads to these items sometimes not being available at all, which would stink.