Selfishness is when we pursue gain at the expense of others. But God doesn't have a limited number of treasures to distribute. When you store up treasures for yourself in heaven, it doesn't reduce the treasures available to others. In fact, it is by serving God and others that we store up heavenly treasures. Everyone gains; no one loses.
The Treasure Principle Bible Study: Discovering the Secret of Joyful Giving (ed. Multnomah, 2010) - ISBN: 9780307563071