Media Mail is an inexpensive shipping method via United States Postal Service (USPS).   At least it was before May 14, 2007.  You will want to check out all rates for your item to find your best option.  It's not for any type of media.  It's specifically for:  books, film, manuscripts, printed music, sound recordings, printed test materials, play scripts, printed educational charts, loose leaf pages & binders of medical information, video tapes and computer media, like CD's, DVD's, and diskettes.   Rates start at $2.13 for up to 1 pound.  But there are restrictions and caveats to consider. 

1. IN A HURRY?  Don't choose Media Mail as your shipping method.  The USPS considers Media Mail to be Bulk Mail.  All bulk mail is sent to a central hub according to the post office of its origin or its destination.  There are 21 Bulk Mail Centers throughout the U.S.  The hub sorts the package and ships according to the USPS time table, not necessarily your time table.  For example, during the holidays, your item could take over a month to be delivered. (I'm not kidding, it has happened to more than one of my buyers.)  Priority Mail is called "priority" for a reason.
2. Media Mail is sometimes referred to as "Book Rate".  The USPS also has rates for "Library Mail" and "Bound Printed Material".  However, these are very restrictive and most likely not available to Ebay buyers and sellers.
3. Media Mail can not contain advertising.  However, incidental ads like announcements of other books within a book is acceptable.
4. The weight limit is 70 pounds for Media Mail.  The size limit is 108 inches total of  length and distance around the widest part of the package.

Media Mail is usually the method of choice for all books.  However, I have often wondered:  Why do buyers who pay a lot of money to purchase a book, then, choose the cheapest and slowest method to ship their expensive purchase?  My theory has always been the longer it's in someone elses possession, the more likely it is to arrive damaged.