I think it would be more useful for users posting listings in the marketplace to include region in the title. Some users list items from multiple regions.
So something like "Stuff for sale [NA]" if you only have NA titles
"Stuff for sale [NA/EU/JP]" if you have titles from multiple regions, in this case NA, EU, AND JP.
So if you stumble into the marketplace only looking for [EU] titles, you can look directly at the title. If you sell all of your titles from a specific region, remove that region from the title... That might be asking for too much, same as asking for users to mark their thread as "SOLD" when all is gone.