The supernatural
The supernatural is an important theme in How to Sell a Haunted House. When Louise's parents die, she goes to their house expecting to easily be able to clean it out and prepare it for being sold. However, she quickly learns that the house has a mind of its own. In fact, the house is haunted and infested with supernatural beings who want to stop Louise and her brother at all costs. The supernatural are an evil force, but they also represent Louise's feelings towards her family and her parents in particular (and how she deals with the grief surrounding her parent's death).
Grief
Although Louise had a complicated relationship with her family and her parents more specifically, Louise immediately starts to grieve the death of her parents. For most, grief is a complicated process. The same is true for Louise, especially because of the circumstances that she finds herself in. When she tries to work through one aspect of grief, which is to clear out the home of her parents, the revelation that their home is haunted and has a mind of its own makes her grief harder to process in many ways. In the end, though, it speeds up her grieving process.
Death
The deaths of Louise's parents are what brings the events of How to Sell a Haunted House into motion. It is never explicitly stated how Louise's parents die, but readers can infer that they have died because of their haunted house. Louise had a strange relationship with her parents, but she loved both of them dearly. As such, she was heartbroken and stricken with grief when she learned of their death. For Louise and everyone in the world, death is a constant. And invariably, death is an incredibly damaging and traumatic thing that people have to experience and go through.
