I've heard a few stories as well.  I dont know 
many offline collectors, but I've heard stories 
from other people who do know them.  The main 
reasons are that they get tired of "sour 
apples".  People who want their cels, but cant 
have/afford them, so they resort to less than 
savory behavior towards them. I know for a fact 
that jealousy can stir up some nasty things in 
this community.  So, I assume most of these 
private collectors dont want to be a part of 
that.  They hide their cels and enjoy them on 
their own, that way they dont have to deal with a 
lot of the bad things that come with a public 
gallery.  

I also know that competition among certain 
circles breeds a lot of hard feelings.  If people 
dont know what you have, they cant hate you for 
having it, or paying what you did to get it, etc. 
In all honesty my life would be easier if I didnt 
have a site, but enough good comes from it that I 
like to keep it up ^_^