If a novel was about a protagonist who used to sell firearms, it might involve the following topics and plots: 1. How the protagonist started selling firearms: The protagonist may describe how he was involved in a political struggle or war and was forced to become an arms dealer. He might describe how he made huge profits by providing weapons and equipment to help and support his clients. 2. The protagonist's career: The protagonist may experience many challenges and difficulties, including struggles with the government, bandits, terrorists, and other opponents. He might find himself faced with many moral and ethical issues in the arms industry and have to make difficult choices. 3. The protagonist's interpersonal relationships: The protagonist may establish his own connections and relationships, including contacts with the government, the army, gangs, and other businessmen. These interpersonal relationships could have a profound impact on his career and even become the key to his success. 4. The protagonist's inner conflict: The protagonist may face many inner conflicts, including reflections on his career, clients, morals, and conscience. He might find himself facing great pressure and risks in the arms industry and need to constantly adjust his strategy and values. 5. The growth and transformation of the protagonist: The protagonist may experience many setbacks and difficulties, but in the end, through continuous efforts and struggles, he achieved self-redemption and growth. He might find that he had learned a lot of valuable experience and lessons from the arms industry and become a more mature and intelligent businessman.