It sounds like you've fallen off the wagon.

The chances are that those Saiyuuki cels are
"Rilezu", or post-production cels, extremely
limited repros which are painted using the
original genga as guides.  New douga are drawn for
the project, and only one of each cel is made. 
You don't get much more limited than that.

Just a theory, though.  Without seeing the
auctions, it's tough to give a definitive answer.