Breast milk obtained online, whether through informal markets or donations, was found to have pathogenic bacteria and other contaminants. These include staph and salmonella, which can pose a significant health risk to preterm and medically compromised newborns. Who, if they need extra breast milk, would probably be getting it from a milk bank, no? Anyway, the American Academy of Pediatrics is not that thrilled about informal online breast milk trafficking, is what they're saying.
UPDATE And as Alison points out in the comments, breastfeeding researchers are not that thrilled about that study either, which collected its samples only from folks who'd send milk with no screening and no questions asked. [i.e., donors or communities who were contacted, but sought more information before sending were dropped from the study.] Also, not all bacteria are bad, and not all other methods of feeding a kid are bacteria-free, so more bacteria than what, basically? Anyway, much to read up on.