I love the idea. Not only because I could put up a shop, but because I could walk around browsing other peoples shops. Sure there is the MB I can browse, but it isn't as exciting as looking for deals all around or simply seeing who is selling what and where.

Another thing you could do to avoid over crowding an area is to make sure people cannot open a shop too close to someone elses shop. It might make people "fight" over spots, but that also could be fun. xD