S.M.
Some pumps are not closed systems and therefore milk/bacteria can get into the maching. I think the Pump in Style is a safe one though not sure. Another thing to consider when buying a used pump is the age and use of the motor. If you are planning to use the pump to maintain a milk supply while away from baby you want to have a good small pumps. Most pumps start to lose power when they are used on a daily basis. So it's good to know the history of a used pump.