Prices from some series raised a lot in Japan. 
Many are now more expensive than outside of Japan.

There is also many follow-up or anime that are 
related to other ones and some japanese collectors 
are suddenly interested again by the original 
serie. (An example that I followed closely because 
I have many cels from this anime is Dangaioh : The 
new TV serie is barely related to the original 
OVAs. In fact, the only relation is that one of 
the main character is mentioned during 20 seconds 
in the new serie. But suddenly, cels from that 
character that were costing 4000 yens started to 
sell for 40000 yen on Yahoo Japan)

A side effect is that I am not sure that dealers 
outside of Japan will continue for long to sell 
cels : Many are costing more in Japan than they 
can be sold for outside. And it's more interesting 
to directly resell back in Japan...

Cheers,
Yann Stettler