Dibs work as long as the people you give them to 
have money when you want to sell.  If they don't, 
dibs go out the window.  To me, giving dibs means 
giving someone first crack at something.  If they 
can't afford it or don't want it anymore, it 
should be fair to move on and sell the item to 
someone else.  I've given dibs to serveral people 
on different cels and I ask if they want it when 
I decide to sell.  Usually, my friends are 
generous enough to let me sell to someone else 
when they can't afford my price.  I've only had 
problems with dibs once when I wouldn't hold a 
cel any longer and the buyer thought I was being 
greedy by putting the particular cel on ebay.  
Situations like that make me not want to give 
dibs on anything anymore.