I like to follow the idea that you shouldn't mix 
frienships with business.  I've been burned this 
way a million times (sold one for $110 and saw it 
end up being sold on his site for $300 about a 
week later -_-).  So i've decided on cels that I 
will absolutley never giveup and cels that I 
couldn't give up except for a certain price (or 
more ^^).  So no matter how much my friend begs, 
I simply state that its my policy and this is the 
price I want to sell at or sorry I wont sell it, 
its a permenent resident to my collection.  This 
way, even if I do see it sell on ebay for 10 
times the price, I'll still be happy that I sold 
it at a price I wanted to sell it at.
Seems to work a lot better for me rather than 
having to deal with the moral dilema of "dang I 
shouldn't have sold it", "wow I feel bad about 
crushing this persons chance at a dream (cel)", 
etc.
My 2 cents,
Hope it helped,
-Eric