It is becoming more common to see whole sim/region land put up for sale by other residents.
On the mainland there is very little room for misunderstanding especially if (one hopes) the purchaser has read and understand the FAQ on land purchase published by Linden Labs.
However room does exist for considerable misunderstanding on private island estates. It is possible that two separate sales options are intended
1) The sim owner/vendor is selling the complete sim and will transfer that sim ownership to the purchaser
OR
2) The sim owner/vendor OR A. N. Other avatar is selling the right to live on that land and the purchaser to pay rent/tier to the sim owner.
I personally have been both witness to, and have had involvements in three transactions, one of which was a genuine sim transfer, and the other two potential transactions were just a land sale without sim transfer. What was far more disturbing was that on one of the latter purchases (Sim Owner was (A very Well known Avatar) - the sim vendor was A N Other Avatar) was that the vendor insisted it was a complete sim transfer when it was absolutely not the case.
Given that the amounts of money involved are typically substantial the risk of fraud is very real, and I can see that cases could occur where the sim owner could be a totally innocent partner to a fraud situation could Linden Labs consider....
3) On private Island complete sim sales add a check box (which is only available to the sim owner to use) to indicate this is a complete sim sale and transfer subject to Linden Labs approval and transfer administration
Regards
John