This window may be accessed from the About Land -> General tab. It is used to sell the parcel to others - assuming you have the ability and rights to do so.
The top of the window gives some basic information about the parcel: the name and size in sqm. Following this are three steps which lead to setting the land for sale:
When you are ready with the above, click on “Set Land for Sale”. And remember, once the land has sold, that's that; you cannot change your mind. (However, you can cancel the sale at any time prior to the land being purchased.)