This has the potential of becoming very annoying. When you put something on the MB you now compete against MB on other servers as well, and if you want to look at their prices you have to jump around among all the servers... well I guess we will need to use an external site in order to save some time on this, which will screw over console players that does not have a pc close at hand.
It would make more sense if they make the MB cross server at the same time they add this feature.