Act as a seasoned marketplace entrepreneur with 10+ years of experience in launching and scaling platforms like [Etsy, Airbnb, or Uber]. Provide a step-by-step guide on how to launch a successful marketplace startup, covering [niche selection, vendor onboarding, and user acquisition]. Include actionable insights on [monetization strategies, trust-building mechanisms, and scalability challenges]. Tailor the advice for a [B2C or B2B] model, highlighting key differences and best practices. Use real-world examples and data-driven recommendations to ensure practicality.