What is the difference between land on a private estate, and land on the mainland?

The difference between them is that 'mainland' land is run by Linden Lab and associated tier fees are paid directly to them. Mainland is originally purchased through an open auction system and then generally resold to the public in world by the auction winners. Private Estates are purchased by individuals or companies and while Linden Lab owns the servers, the estate owners have control of the land and can do with it whatever they wish within the Terms of Service rules. You must be a Premium member to buy land on the Mainland, however you may rent land on the Mainland or an Estate, or buy Estate land as a basic member. So, the mainland is run by Linden Lab, and private estates are run by whoever leases them from Linden Lab. 

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