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. ;)

Izzy