Comic arts are highly collectible and they’re not cheap. For instance, the original concept art of Wonder Woman by H.G. Peter dated 1941 was sold for $75,000.00 at Comic Link.
The same should hold true for Japanese comics, right? Maybe, if there are manga artworks being traded. Well, there really are manga artworks listed for sale at Mandarake but not from bigshot names like Akira Toriyama and Inoue Takehiko.
So, why are original manga arts not for sale? I found the answer at Masters of Manga.
Basically, in Japan, when a mangaka sells his work, what he’s really selling are the contents. He retains ownership of the materials that hold the contents, which means retaining copyrights. So selling the original artworks is akin to selling the contents, therefore relinquishing ownership.
Here’s a more detailed explanation from Kenshi Hirokane, mangaka and president of Manga Genko o Mamoru-kai (Association for the Protection of Manga Originals):