Buying a used breast pump or sharing a breast pump is neither safe, nor recommended. For lactation consultants, the question arises every day: "If I buy my own flanges (the part that is placed on the breast), bottles, and new tubing, why can't I just borrow the actual pump from my sister (friend, co-worker, the list goes on...)" Here's the reason why:
Believe it or not, it's not the parts that touch the breast that are of concern. It's easy to purchase new flanges, bottles and tubing. The issue is the pump itself. Only FDA-cleared, hospital-grade pumps should be used by more than one person; they have filters on them, and the tubing is not fed directly into the motor. This is also called a "closed" system. The FDA considers personal breast pumps single-use devices, unless deemed hospital-grade, as there is no way to clean and disinfect them between uses by different women. This is the reason why it's safe to use a rental pump, which is always hospital-grade.
It doesn't matter how well you know the person who is offering her personal pump to you. Because personal pumps are "open" systems, where there isn't a filter, tiny milk particles can back up into the part of the motor which causes the suction and release, and fester. Unlike a hospital-grade pump, a personal pump is impossible to sterilize, hence no guarantee of safety can be made to the next mom who uses it. If milk particles from the previous owner are on that section of the motor, they can be blown into your milk, and the potential for diseases such as HIV or Hepatitis become the bigger issue. The FDA states,
"There are certain risks presented by breast pumps that are reused by different mothers if they are not properly cleaned and sterilized. These risks include the transmission of infectious diseases...FDA believes that the proper cleaning and sterilization of breast pumps requires the removal of any fluid that has entered the pumping mechanism itself. If proper sterilization of the breast pump cannot be achieved, FDA recommends that it not be used by different mothers."
Food and Drug Administration. Choosing a Breast Pump.