Maybe in the US demand is low, but good Metropolis go for big money in Japan.
The cels of Tima are rare since many of the good scenes are computer generated 
only.  It's difficult to find good cels of her.

Actually, it isn't Madhouse, but Film Link who owns the D cels.  Madhouse has 
nothing to do with it except being the creator of the film. 

Also, Ninja Scroll was never released.  Most of the cels were thrown away and the 
cels you see now are just the ones that managed to not get thrown away.  That is 
why you won't see many Ninja Scroll cels at all.

And with D, many of the nicer cuts were kept by the staff, so it is very hard to find 
good cels.  Also, characters like Carmilla, nice up shots of D and Caroline are 
almost non-existant.  The do not appear that often in the movie and those that do 
were ones kept by the staff.