I wasn't sure who you you're aimin' this question 
at, but as far as my collection goes, I buy cels 
because I am profoundly fond of the anime and art 
of certain series!  The reason I sell, whether I 
lose money or not, is only so I can buy a cel I 
like more and keep my collection alive in 
general.  If I could afford to, I would keep many 
cels I have sold in the past, but I often can't 
afford to do that and buy new cels (especially 
really 'spensive ones).  I think we all at least 
LIKE to get what we paid for something, and 
there's nothing wrong with that in my mind, 
especially is the buyer really enjoys and 
appreciates what you sell them as much as you did 
when you acquired it.  
As far as the number of cels in my collection, I 
have some expensive cels which mean a lot to me 
aside from the cost.  I only have what I can 
afford to have, and a small, satisfying, and 
personally meaningful collection is far more 
preferable to a large collection of mediocrity... 
in my opinion.

Cheers  ^__^
Ghost'