To help you manage expectations, many sellers use a grading system: Grade A: Near-mint condition; may have very minor scuffs.
Most go through a manual diagnostic process more rigorous than random factory tests. Key components like hard drives, batteries, and RAM are tested and replaced if they don't meet standards.
Unlike "used" computers, which are sold as-is by individuals, refurbished devices are returned to a manufacturer or certified vendor, professionally tested, repaired if necessary, and re-certified for sale.