Medela Pump in Style is not recommended for second hand purchase. If you read the manual carefully, it seriously warns people "not" to buy a second hand one because not all parts can be fully santisized or replaced with new.
Unless you "really" know the person who owned it before (like your own sister), then do "not" buy used one, or you are risking your child's life to unknown diseases that the previous owner carried, for just saving a hundred dollars or so, which is absolutely not worth it.
Another reviews stated this below:
*Warning: This is not something you can find cheaper by buying used. This is a medical device, please don't buy it second hand. You really don't know what types of diseases the last person carried... and even if you buy new tubing and shields, old milk could have gotten into the diaphram itself... meaning you could still contaminate your milk.
"Medela and the FDA warn about second-hand pumps becoming vectors for the transmission of HIV and Hepatitis. "