Mandarake Zenbu is a catalog that Mandarake puts 
out every few months.  It contains not only cels 
but other collectibles like books, comics, toys, 
and sketches.  Each item is priced.  If you would 
like to buy an item, you fill out the form that 
comes with the book or online (you have to buy 
the book to be eligible).  Mandarake then has a 
drawing (lottery) to determine who of the people 
who sent in gets to purchase the item.

Here's an advertisement page for the last 
Mandarake Zenbu, it might make things more clear:
http://www.mandarake.co.jp/english/publi
sh/zenbu_20/

Hope this helps!
Keys