Hi Pale.  ^_^

In my experience, I've actually gotten some 
decent cels by asking private collector's if they 
were willing to part with them.  The best way to 
ask (like max said) is to make an offer.  

Saying "wow, that cel is nice, how much would you 
want for it"  is a bad idea.  Most people don't 
like that - I don't either. My reason is that I 
wasn't planning to part with it, why should I 
make up a pricetag?  If someone makes an offer 
that makes me consider selling it, then it's a 
possibility.  

Also, you could consider offering a reasonable 
trade.  If you have something on their wishlist 
it also helps.  ^^; 

The worst that can be said is "no".  If they do 
ever consider selling it in the future, they 
would know who to offer it to.  ^_^

Good luck!  

jINx