When sourcing used soap machinery, you must prioritize the mechanical integrity of the plodder (extruder) and the milling rollers. Ensure the vacuum pump in the duplex vacuum plodder maintains a consistent pressure of -0.08MPa to -0.09MPa for high-density soap bars. Check the material of construction; for long-term durability and hygiene, Stainless Steel 304 is preferred over carbon steel. Additionally, verify the motor power (kW) and gearbox condition, as these are the most common failure points in second-hand equipment.