I subscribe to "quality over quantity". When I
find a cel of a character that makes me
happier than one I already have, I'll usually
sell the lesser cel.
I've also perfected what I call the "guilty until
proven innocent" rule. ^^ Whenever I see a cel,
I purposely try and find all the things that could
possibly bother me about it. If nothing stands
out, I buy/keep it, because I'm picky enough to
know that if anything really bugs me about a
cel, it's going to get worse, not better, over
time and I will eventually end up selling it.
So in short, you should only keep cels that fill
you with satisfaction every time you see them.
Hope this makes sense. ;)