Act as a seasoned entrepreneur with 10+ years of experience in e-commerce and subscription-based businesses. Provide a comprehensive step-by-step guide on how to start a successful subscription box business. Cover the following key areas: [1] Niche selection and market research, [2] Sourcing products and building supplier relationships, and [3] Marketing strategies to acquire and retain subscribers. Include real-world examples, common pitfalls to avoid, and actionable tips for scaling the business. Tailor the advice for a [budget range] and a target audience of [demographic]. Also, address how to leverage [specific platform, e.g., Shopify, social media] for maximum growth.