Personally I'd check out the art galleries at cons and just look to see what is selling well and ask "why". Visit the auctions too. And try to talk to those artists as well.
I would imagine not just Inyuyasha but most widely recognizable or popular anime art would also make for popular purchase pieces. You can only own so much "official" stuff so those fans start to crave and look for their favorite anime from other sources, including fan art. Not that original artwork wouldn't sell well, but I'd bet original artwork would be easier to sell after you establish yourself as a known artist. Also with original artwork I think you have more pressure to come off as unique in style, not just in subject.
Of course I'm just speculating here, I have no practical experience.