Mark O'Brien flips brownstone apartments in New York City, buying distressed properties for $800K–$2.8M, investing another $800K–$2M in renovations, and selling for $2.3M–$6.2M. Sam Parr spends 24 hours with him to evaluate the economics and lifestyle of blue-collar real estate development versus tech entrepreneurship. Mark's brownstone business generates approximately $500,000–$1 million annually but requires 2–3 years per project with significant regulatory delays and risk of total loss.