A few years ago, I was in one of our local Christian bookstores. This one has an aisle of Catholic stuff, including a set for people trying to sell their homes. The kit contained a statue of St. Joseph and instructions for burying it upside down to sell your house. Superstition? If a person believes planting that guarantees a sale, then yes, it's superstition. Planting a statue, even upside down as you're "supposed" to do, does not guarantee the intercession of St. Joseph and certainly does not bend God to your will. It is far too easy to use this as magic.
It doesn't have to be that way, though. The same statue can be a reminder of God's protection, particularly as it played out in the life of St. Joseph. St. Joseph lived his life seeking God's will. God responded through more direct intervention than most of us will get! St. Joseph followed God, even when it was difficult. As a provider for his family and exile for a number of years, St. Joseph may well be inclined to intercede with and for us as we seek God's will for our families.
Trusting God's protection in all circumstances is a valuable lesson. Learning to see God's providence in a situation (such as the sale of a house) is certainly worthwhile. If a physical act like planting a statue helps you learn that, go for it.