One of the things that I look at when selling a 
favorite cel, is how rare it is- A couple of 
times I sold a cel I really liked to a friend, 
bought the new, different cel, and then a month 
or so later when I had money, bought another cel 
from the same sequence as the one I sold.  (just 
so happened nice Ayako headshots were pretty 
common at the time.  ^_^)
Another thing to look at is how many/what quality 
of the cels you have are, if they are of same 
character especially.  I have to keep reminding 
myself that I really can be content with less 
than 5 headshots of a favorite character. ;)
I usually sell "lesser" cels of a character to 
get better ones.  (example- sold 2 or 3 yohko 
cels to get one really nice one of her).
For another example, I somehow ended up getting 5 
or 6 nami cels from iczelion, now I'm selling a 
couple of those to get a different cel from a 
series I only have 1 cel from.