When sourcing used equipment, the engine hours (running hours) are the primary indicator of remaining service life; ideally, look for units with under 5,000 hours. You must verify the CFM (Cubic Feet per Minute) and PSI (Pounds per Square Inch) ratings to ensure they meet your pneumatic tool requirements. Inspect the air end (compressor block) for signs of oil leakage or abnormal vibration, and request a video of a cold start to check for excessive smoke, which could indicate engine wear or fuel system issues.