Acetitis.  After a certain point, cels kept in 
close proximity in a high population density 
environment begin to cohabit and subdivide, 
producing new cels that you have never seen 
before.  Once this begins, it can continue 
gradually or explode exponentially.  Be aware 
that some cels need energy to keep from fading, 
which is why your electric bills have been higher 
recently. If you don't do something about, you 
may even have to move out of your house because 
the cels will take it over, answering your phone 
in your absence, bidding on more cels while you 
sleep, accessing your credit card and Paypal 
accounts, etc. etc. 

Happily I run a charity that will help you free 
of charge by reducing the density of cels in your 
house to healthy size.  Sending me 4 small cel 
books and 4 large cel books should just about do 
it and make your life much happier.  (You can 
choose which to send me, but a word to the 
wise ... CLAMP-related cels and sketches are the 
most prone to unbridled cohabitation and 
replication.)

Seriously ... this is a problem better handled 
online than in the books.  Probably you're better 
off just putting the cels in the order that you 
get them, and give each one a "shelf list."  

One suggestion: give each book a name, and then 
the cel will be number six in that book.  So if 
you use character names, your new cel might be 
Sumomo 6. (Or if you're boring, you can just use 
letters and call the location A6, but then that 
looks like a sequence number and might be 
confusing.)

Then scan them (as you do ... beautifully, I must 
say) and organize them virtually (as you do, 
nicely).  Then all you need to do is include 
the "shelf list" somewhere in the description and 
you know exactly where the cel is.

The higher electric bill, strange phone calls, 
and YJ bids you don't recall making remain a 
problem, though.