On these three platforms, thousands of sellers offer books. Some are large book sellers, others are individuals selling a few copies of each book, or perhaps one copy of a book. Prices can vary substantially, if you happen to find the person who wants to sell his book for a low price.

Amazon - there is a link, under the book, or the buy box, that says More Buying Choices - Used and New Offers, click there to review other vendors offers.
AbeBooks - many vendors offering different prices, some with free shipping
eCampus - buy, rent or sell books. Also can buy rent or sell books back after the semester on Amazon.

Medical Textbooks on Amazon - Rent, Buy, New and Used

Doximity - social network for doctors https://www.doximity.com
figure1.com -upload images, discuss real-world medical cases with healthcare professionals http://figure1.com
Case Medical Research https://play.google.com/store/apps/details?id=co.kernelnetworks.casejournals&hl=en
Read by QxMD - read & research medical journal https://play.google.com/store/apps/details?id=com.qxmd.readbyqxmd&hl=en
Open Science Directory website - access to more than 13000 open-access medical journals, articles, and databases http://www.opensciencedirectory.net/
MDCalc - 350 decision tools - learn and apply evidence-based medicine
website and app for phone
Prognosis : Your Diagnosis - 400+ cases to diagnose and manage key clinical presentations https://play.google.com/store/apps/details?id=com.medicaljoyworks.prognosis&hl=en