It has to do with the fact that it isn't sanitary, since all parts can't be sterilized.
This from another board:
Medela, Inc.:
" 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. Breastfeeding is certainly the best way to feed your baby, and is the gold standard of infant nutrition. There is some evidence, however, that certain serious viruses* may be transmittable through breastmilk. For this reason, it is not advisable to use a previously owned breastpump. Breastpumps are single-user products, or personal care items, much like a toothbrush, and are registered with the FDA as single user items. For safety, breastpumps should never be shared, resold, or lent among mothers. Medela strongly discourages mothers from re-using or re-selling previously owned breastpump equipment. The Medela Pump In Style Breastpump 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 totally dismissed. Multiple use of single-user breastpump automatically voids the warranty of the Medela product. Each mother who wishes to express milk with a pump should use a clean, uncontaminated breastpump. This is the safest way to eliminate any risk of cross-contamination. Rental pumps such as the Classic‰ and Lactina pumps are made to be safely used by repeated clients who each use their own clean personal rental kit, therefore avoiding any possible cross-contamination. Rental pumps, when used according to the Medela instructions, are safe to use by multiple mothers who have their own personal kits."
Avent America:
"We at Avent America are always striving to provide mothers with quality products at reasonable prices to help them breastfeed longer. Research has shown that breastmilk can transmit many contagious viruses. It is for this reason that we strongly recommend that you NEVER use a previously owned breast pump. The Isis Breast Pump is considered to be a personal care item and has been designed to be for single use only. Mothers should never share breast pumps. Sharing or using a previously owned breast pump could put you and your baby at a potential risk for exposure to serious health risks. Some of the viruses that can be within breast milk are: HIV - Human Immunodeficiency Virus (AIDS) HTLV-1 Human T-Cell Leukemia Virus Type I CMV - Cytomegalovirus When you are using a previously owned breast pump you create the risk of cross contamination. It is for this reason that AVENT AMERICA STRONGLY SUGGESTS NEVER USING, BORROWING, PURCHASING OR SELLING A PREVIOUSLY USED/PRE-OWNED BREAST PUMP. Since a mother's breast milk is the most precious gifts of nutrition/health she can give her baby, DON'T take the chance of sharing someone else's viruses with your baby. "
So I can't say I really think this is a bad thing.