Should I be mad about this?

My family had a tag sale today and my dad said I could sell my stuff and get the money from it, but one of our neighbors came along and wanted to buy some of my old books and my dad made me give it to him for free just because he lives near us. I feel like it's MY books, MY money, and I should have the right to set the price. I would have given him a discount just to be nice, but no way was I going to give it to him for free! My dad made me and I'm so mad!