This sort of thing did happen to me, I had sold a 
bunch of cels to buy my dream cel- the seller had 
promised to give me a week to come up with the 
money- then when I asked for her address to send 
the money, she said the cel had a crease and she 
couldn't sell it to me.
Someone else showed me the exact image of the 
same cel in their gallery a year later.  
Obviously, this still bothers me.  I have tried 
to sell my cels to nice people that I know want 
them and will enjoy them, but I haven't ever 
promised a cel to someone then cancelled their 
order for someone else.  If I am thinking of 
selling a cel, I will sometimes take a higher 
offer, but certainly not after promising it to 
someone else.
Of course as a seller, things get sticky if you 
put a cel on hold for someone and don't hear 
anything for weeks- in that case then I could 
understand the seller cancelling the order- no 
updates from a buyer and no money obviously 
change the situation. But it doesn't sound like 
that happened with you- I'm sorry to hear it and 
I know exactly how you feel.