This is off the lactation specialist site at Medela.com and is the same as the handout my daughter got in her class.
"Many mothers have asked if they can safely sell, purchase, or use a previously owned breastpump. Medela is concerned about the health and welfare of breastfeeding mothers and their babies. There is evidence that bacteria and certain viruses may be transmittable through breastmilk. For this reason, it is not advisable to use a previously owned, personal use breastpump, such as Medela's Pump In Style®. However, it is safe to use a Rental pump, such as Medela's Symphony®, Lactina™, or Classic™ pumps. The difference between personal use pumps and rental pumps is as follows:
Personal Use Breastpumps
Personal use pumps that you buy at the store are personal care items, much like a toothbrush. Personal use pumps should never be resold or shared among mothers. The Medela Pump In Style has an internal diaphragm that cannot be removed, replaced, or fully sterilized. Therefore, the risk of cross-contamination associated with re-using a previously owned pump such as the Pump In Style cannot be dismissed, even when using a new kit or tubing. Similarly, the Medela MiniElectric™, Single Deluxe™, DoubleEase ™ and Double Deluxe™ Breastpumps have internal motors that cannot be removed, replaced or fully sterilized. Multiple women using these single-user products automatically voids the warranty of the Medela product."
Now that being noted from the manufacturer, I do not remember the last time that we took apart the internal motors of the 10 pumps we clean with antibacterial solution (external parts) and cart from room to room in our facility and give with new disposable parts to our new moms. Is this just CYA from medela?