It's a buyer's market right now, and probably has 
been for at least the last year or so. I have 
cels that I bought for $30 which are lucky to 
sell for $10; but I also have friends who bought 
cels for $50 or $100 which go bid-less. So 
there's not quite anything as clear-cut as a 
percentage you can use as a rule. It just helps 
in general to be familiar with the series, and 
what the market has been supporting, and price 
your cels accordingly.

But there are other factors. A lot of the people 
who have been collecting cels (and who have 
contributed to some of the big bidding-wars in 
the past) have more complete collections now and 
are content to let nice-but-unneeded cels float 
past. Without the competition, a buyer can get a 
decent cel at a stellar price, but it's bad for 
the seller. The lack-of-competition might come 
from other areas than a complete collection-- 
marriage, buying a house, pets, unsteady jobs, or 
even just wavering interest in cels in general.

Likewise, it seems that anime is becoming more 
and more mainstream, and more and more of these 
new fans are younger. Less attention is being 
spent on "older" shows, and more attention is 
being spent on the new CG shows. However, there's 
less of a following for individual shows... the 
fandoms seem much more weakened than they did 
before, so there's less of a drive to collect 
production art or paraphernalia related to a 
series. 

There will always be a niche for production 
art... but the people who buy it haven't been 
collecting the same way they used to.