As a Registered Nurse you can imagine I've picked up volumes of reference material to add to my home library. You can never know everything, and having a book around to guide you through specialty changes and new procedures helps immesurably.
Something I would like to stress to would-be sellers of medical journals or textbooks is that information changes very rapidly in the medical field. For example, a drug that was popular last year may be pulled from the market this year. Drug guides and medical texts are extremely expensive for the simple reason that they have a limited shelf life. During your travels, lets say you find a medical text from the local university - the original $45 price tag is still on the book! You may think to yourself that there is money to be made reselling the book, but take the time to look at the publishing date - is it more than 2 years old? If it is, then leave it there! You'll have a very hard time getting rid of a textbook that has outdated information or that is no longer being used in the classroom.
Almost all students in the medical fields need to buy textbooks, but the fact is we were told exactly what to buy, and they are available at the campus bookstore. If you can find a textbook that is only a year or two old, you will probably sell it to an aspiring practitioner looking to beat the bookstore prices. However, the amount of money we must spend on books is usually so great that it precludes buying anything that isn't on the list.
Some older books that will sell are specialty books that address specific procedures. I have great books on interpreting EKGs and interpreting X-rays, both from the 80's. Books on topics like "Spanish for Medical Practitioners" will always be handy.
The last thing I'd like to mention are vintage medical books. I quickly snatched up "Ethics for Nurses" from the 1920's, and "Modern Medical Advisor" from the 40's in my garage sale rounds. There is probably a reasonably decent market for books like this if they are in good condition and on an interesting topic, like childbirth or surgery.
There are also collectors of antique medical tools, nursing badges, pins, and such - so keep an eye out for those when you start to see medical books!
... and always treat your nurses nicely ;)