There isn’t much of interest on eBay today, unless you are looking for Seiken Densetsu books. Seller japan-ssdyuk always has an awesome collection of Mana books and probably any other Japanese book you could want, and right now pretty much everything is on sale! Also, everything listed by this seller has a “make an offer” option, so you could possibly get stuff for even cheaper than it appears!
Here is a list of the Mana books with the best deals:
Complete set of Original Legend of Mana Manga
Complete set of Re-released Legend of Mana Manga
Seiken Densetsu Guitar Tabs
Seiken Densetsu Piano Scores
Sword of Mana Manga
Sword of Mana Four Panel Comics
Making of Mana Legend of Mana Artbook
And finally there is the CoM and DoM Artbook. Please note that 75 dollars is WAY too much for this book, but if you can take advantage of the “make and offer” and try and get it for around 30 this would be the cheapest way to get the book because shipping for this book is usually around 18 dollars and here it is only 10.