Why does Shiroe buy all the land in Yamato and give it to the server?
The Round Table cannot afford to hold onto the buildings forever so they are going to lose control eventually. This means another guild could use those buildings against the players of Akihabara. By buying all the land and making it unpurchasable for the future this problem is solved forever even if it is only a middle of the road solution.

Additionally, Shiroe isn't just concerned about players. Before he bought everything and then gave it to the server, the NPCs couldn't buy zones but players could. This means the players could buy a farm under an NPC's nose and force them out of their home. This would definitely lead to war with Eastal. Now property ownership is an agreement between people and not a function of nature.

As for people going wrong and exploiting others, this is no different then the real world. Most of Akihabara has learned to live in this world and is not treating it like a game. Additionally, the Round Table can create licenses to allow open trade in Akihabara, and since most would follow that order then not being able to trade with other people is enforceable. They just have to create new systems to organize and punish rather than relying on server systems.