When sourcing used outboard motors, you must prioritize the engine hours, as this is the primary indicator of remaining service life. Ideally, look for units with under 500 hours for recreational use. Verify the compression ratio across all cylinders to ensure internal health; a variance of more than 10% between cylinders is a red flag. Additionally, confirm the shaft length (Short, Long, or Ultra-Long) to ensure compatibility with the buyer's boat transom, and check the tilt/trim system for hydraulic leaks or corrosion.