Sounds like a pretty common mistake. I have sold 
things with backgrounds and had sometimes 
forgotten to pack them when I sold them. Like you 
said, an honest mistake. However, when I was 
alerted to these mistakes, I made reparations to 
settle the matter to both our satisfaction. This 
person you bought the cel from seems to be 
unwilling to do that for you, so this honest 
mistake has not become fraud. Since it is a 
personal seller, I think you are just going to 
have to take your lumps on this one and be glad 
you have a nice cel. Plus, now you know what kind 
of person you are buying from and will be a bit 
wiser the next time. Sorry about your situation, 
and I certainly hope things get resolved, for 
both your sakes. One thing about cel collecting 
is that when you screw someone, the whole 
community finds out about it pretty darn quick 
and your reputation is tarnished until you make 
it right again.