What does it mean to buy a house and sit south to north?

"sitting south facing north" means facing north with your back to the south.

in the house, facing the door, the direction of the house is the direction you are facing at this time. This is called "Xiang" and the location of the house is called "Zuo". Then the house facing south is the house facing north and the door facing south.

Extended information:

The orientation of houses is the saying of ancient traditional buildings. Most of the houses in China are built in the form of quadrangles, in which the orientation of each house represents the position of the house. The following methods are used to judge the orientation of the house.

Generally speaking, the orientation of houses is mainly for the need of lighting. Generally speaking, bungalows in the northern hemisphere face south. This is conducive to lighting, can maximize the acceptance of sunlight.

Reference: Housing Orientation _ Baidu Encyclopedia