You will for sure need a huge standalone website for this at some point. I have been working for a few hours on it and I think you will get to see how we could feed that data. You could keep breaking it down into areas as the need arises. FYI we can post to Steemit from SteemThat.com so I might be able to make a small box in a side panel where people could create there post while looking at others posts. This would be very unique. Whatever area a user was on Like France, Or Korea, Or Australia the form would auto generate the correct tag so no worries with that. Steemit would hate it because of an extra million broadcasts per day though..
making posts through SteemThat on the sidepanle could be a thing. Or choose your flag and give them a list of little flags and when they select their flag the script then would create a tag from that? I could write a list of all the countries in the world with all possible #steemfleamarket-(insert country) tag if needed :)