I'm not sure this is a very good idea...
If I'm understanding you right, it'd be a server/map where you'd log in and could modify your character instead of going to the site?
As in, we go to the site for the marketplace, your character would run to the marketplace on this specific map/server and then get an in-game marketplace which was the same as the one on the site?
Same for equipment...
Yeah, I mean, I guess it would be kind of cool but seems complicated for things we can already do easier.